1996 Passat B4 heater core replacement, HVAC foam replacement, and air box rebuild
In all 1990s Volkswagens (according to a quick look through the aggregated wisdom of the TDIClub and VWVortex forums), you need to be prepared for a certainty that accompanies ownership: the foam gaskets within your heating and cooling system were open cell foam, and thus will eventually rot away. More specifically, they will rot away and shoot out your vents at you (it is charcoal grey foam)...
...and in my case, also get into your A/C evaporator drain and cause the condensation to back up and eventually drain into the car. Not a good outcome. I used a variety of wisdom from the TDIClub forums, and this post about Passat B3 heater cores by IZZO, to help me make accurate use of my Bentley Books to gain the best understanding of the projects at hand.
Please note: other guides will suggest you don't take the dashboard out fully. At first, you'll agree with that as this is a lot of work BUT: you cannot access and thus cannot fix the most important of the foam gaskets for safe winter driving - that on the air box flap which directs air to or away from the defrost vents in the dash shell. By failing to replace that, you'll continue to experience weak air flow to the windshield, and thus the slower (or non-functioning) defroster will continue to haunt you.
People have made the claim that this is the most difficult project you can do on a B4 - having done it, I can agree, due to a combination of the tediousness of removing all the components, the fear of breaking 20 year old plastic connectors, and forgetting to reinstall something and having to take it all apart to make it right.
Do this once, and do it right.
Along those lines, be forewarned: this is a lengthy and detailed post.
Parts and Tools
1) Which heater core? The most important question here, in my mind: if you've got the dash apart and are going to replace the heater core, do you want the perfect fit of replacing it with the same Valeo plastic tubed model... or are you willing to do extra rebuild work for the far more reliable and longer-lasting Behr all-metal, Passat B5 heater core?
*Trade-off with the stock Valeo model (Spectra sells one, not sure how it works): you guarantee a perfect fit into the firewall, at the cost of putting in plastic coolant tubing which will also eventually become extremely brittle with age
*Trade-off with the Behr B5 model (part number 8D1819031C ): you get the satisfaction of almost certainly never having to replace the heater core over the life of the car, at the cost of the angle of the coolant tubes being a bit higher than stock, necessitating you fabricate your own coolant flange on the firewall.
I chose the B5 core, it is on the left below:
1b) Fabricating your own coolant flange
If you choose to use the B5 all-metal core as I did, you'll have to make your own coolant flange on the fire wall. I explain this below in the Procedure, but here are the parts you will need:
*the remainder of the McMaster foam (see below)
*high temperature RTV gasket-maker (its the red one)
*.3 inch aluminum shim (this is what I used, other gauge metals might be better)
*to cut the aluminum you'll need a jigsaw and a bi-metal carbide blade
2) Redoing the HVAC foam the right way. Again with the "do it once, do it right" philosophy, I used the McMaster-Carr high temperature adhesive-backed foam for the majority of this project.
*x1 12x12" square of extreme temperature adhesive-backed silicone foam
3) Rebuilding the air box. Besides the all-important foam above, you're going to want to do some mechanical work too.
*Lucas Red'n'Tacky #2 grease for the blend, mode, and defrost doors (and also the gears on the HVAC controller)
*Tri-Flow industrial lubricant, for the cable controls from the HVAC dials to the air box itself
*Nashua Extreme Weather Foil HVAC tape
4) other tools
*spring clamp tool, to remove the coolant tubes from the heater core
*flat head and Phillips screw drivers
*Torx bit set
*24mm socket - for the steering wheel
*Allen wrench set (steering wheel airbag, other tasks)
*cordless drill with a set of drill bits
*10-24 machine screw and nut (any medium size will do, to replace the rivet you drill out within the dash shell)
*good lighting, including a head lamp
*McMaster-Carr 1/2" adhesive compressing foam
In my experience, all of the above are completely necessary. The following tools and parts make your life a lot easier but aren't technically "required":
**Leatherman multi-tool. This almost makes it to the required parts list in my opinion, because it is so useful to have in your pocket rather than getting out of the car... again... to get another tool... again.
*McMaster highly compressible foam (part number forthcoming)
*Fiskars titanium circular cutter. I am a very proficient knife user, but the ease of cutting the foam without worrying about extraneous tears from this cutter made it worth the price.
*replacement silicone tubing for the blower motor. This affects whether your recirculation door works or not, and will be something you mess with when removing and reinstalling the coolant flange from the firewall (where the stock tube passes through)
*screwdriver (de)magnetizing tool - this will make a lot of the project, but ESPECIALLY the heat box clips, a LOT easier to work with, as it is a struggle to get your hand to those places.
*the smallest ratcheting screwdriver you can find, for a couple of incredibly annoying screws on the lower parts of the interior panels
A blanket statement true for each step that follows: LABEL EVERYTHING CLEARLY BEFORE DISASSEMBLY.
A general outlook for you, from the Bentley manual:
All told, in my experience the worst part of this job is keeping track of (and having room for) all of the various parts involved. My strategy was to use masking tape and permanent marker to tape screws to the vent or interior piece they are associated with, AFTER labeling the tape with "front left heat duct" so I could quickly and easily see what it was (if merely looking at it didn't instantly associate it to a location for me). Good news: the electrical connectors are ALL different shapes and can only fit into one other electrical receptacle, meaning you won't forget to plug anything in *IF* you are slow and make sure to plug everything in as you put it all back together.
I am going to number this list in bold, so when I say "reinstallation is reverse of the above," you can make SURE you do all the steps in order and not have to take it apart again because you forgot a part. This is particularly annoying when it comes to reinstalling the bezel atop the instrument cluster - all sorts of HVAC duct and other components go in behind it, first. But I get ahead of myself; first, focus on...
1) Setting up the car as your work space.
This is actually worth stating - it will help you a lot to have a laptop or printout in the car with instructions and related information. In my case, I have the entire interior of the car out for everything from welding in two patches over holes in the underbody; to redoing the rear suspension and thus needing access to the rear wheel wells; to planning on installing soundproofing the whole car across:
You won't be able to get the benefits of a 500 watt halogen work lamp without removing the passenger seat, but that is your call. If you keep the seat in, I would cover it with an old towel as the old foam shards easily get ground into anything they touch.
** BE SURE TO DISCONNECT THE BATTERY FULLY BEFORE TOUCHING AIRBAGS **
2) Removing the steering assembly.
Not every step here will have a photo, but I can explain them in order.
a) Assuming you obeyed the above and fully disconnected the battery, I would start by unscrewing the steering wheel airbag. They are hex screws on the underside/back of the steering wheel. Once loosened they stay in place and the airbag lifts out. It is heavier than you might expect (do not drop it. Do not.), and it is connected to a wire harness plug you just pull straight out.
b) The steering wheel itself is a 24mm bolt which should move extremely easy - not torqued down too much as that inhibits free wheel movement. The clock spring is the flat plastic cylinder on the back of the steering wheel, TRY not to break it open like I did. You need to carefully lift the wheel straight off. There are a few plugs you will need to navigate, a yellow one allows you to disconnect the airbag sensor wire and pull the wheel out of the car.
c) The steering wheel clam shell is the black plastic which moves as you adjust the wheel up or down and protects the wiring to the stalks etc. It has two Phillips screws accessible from below and in my case they were a royal pain to get moving. Be gentle and patient, do not strip them.
At this point you can begin to see how everything is nested and needs to be removed in order:
d) The stalks have nested wiring harnesses. The turn signal has 2, the wiper arm has three. Be gentle with them as well, as they are seated in old plastic tabs which you need to flex slightly to remove them. I was careful to label all of them initially... but they are each a unique size and configuration, so you can only put this back together one way. You will need Torx-10 bits to get the three long thin screws out to get them off the steer column.
e) Make sure you tape the screws to the pieces they held together and label them if necessary - and do not drop the airbag.
3) Removing the headlight controls and the radio/HVAC controller/etc
Note: you will need to remove the lower plastic piece of the center console, which I did a month prior to this project, so I don't have photos of it. Refer to elsewhere on this site, or the TDIClub forums.
Next up will be removing all the things plugged into the dash shell - after you do this, you will understand why I say "plugged in."
a) The headlight switch has a pair of grey studs coming out the bottom of it, which you can manipulate in a variety of ways to the right to remove it (I used the flat back edge of my Leatherman knife, there are likely better ways). It pulls straight out, and you can remove the wiring harness from the back of it.
b) The four upper vents are easily popped out, I used the edge of my Leatherman knife to start the one but realized you can just tilt the vent all the way up to get a grip on its edge, push it one way and then the other, and it will come out.
c) The leftmost opening or button under the radio is either a blank cover or the heated seat controls (if you're someone who has them). This is my suggested point of attack to begin disassembly of the center console. From it you can unseat its righthand neighbor, the rear window switches. Be sure to label each if you're worried:
These are relatively easily removed, and you need to remove the wiring harnesses from the back of each. It is not worth bundling them together at this point because you need them easily moved, to remove the dash shell.
d) I would suggest removing the radio next, which can be done without the special grabber tools (used to unseat it from the rails it slides into, I believe) if you reach in from below and pull at the bottom edge. Be careful not to bend any of the pins from its main wiring harness - they are VERY small and fragile.
e) To remove the HVAC controller, remove the bezel first (it can just be gently pulled off). Know that there are a variety of screws holding it into place, and the little green and clear plastic bar is the mechanism which lights the controls - remove all of this carefully. When you have it unfastened, the controller box slides back into the dash - so don't worry about it for now, until the dash shell is removed:
f) There is a general black bezel to the center controls which I removed, just to make it easier to remove the wiring and the HVAC controller.
4) Removing the passenger side airbag is next.
I don't want to be repetitive, but:
** BE SURE TO DISCONNECT THE BATTERY FULLY BEFORE TOUCHING AIRBAGS **
That said and meant, this process is a bit slow but worth doing right.
a) The airbag cover has three screws holding it down, remove them and then you can life the cover up on its hinges.
b) This gives you access to the airbag itself, and it is held onto two metal mounting arms with a total of 4 bolts. Remove them and BE SURE to have a strong grip on the airbag - it is a lot heavier than it looks, and is technically an explosive. You don't want to drop it.
c) The airbag sensor has a wiring harness plug into the back of the airbag itself - remove it carefully (for me, the wires came out of the plastic plug very easily, and you don't want to reinstall them backwards!)
d) You can remove the airbag cover if you're going to install a glove box instead (see below), which is also held in with three bolts.
NOTE: this is a good time to do some soul-searching and deciding if you want to do a Passat B3 glove box installation here instead of the passenger airbag. I have done a write-up of that project.
5) Removing the instrument cluster
a) Remove the L-shaped bezel over the OBDII port, to the right of the ignition. It will expose one of the screws holding the cluster into place. The other screws (6 total I believe) are located along this single bezel - one in the driver's side vent, two along the bottom edge of the cluster, one by the OBDII plug, and the remainder in the central HVAC vent ducts. BE VERY CAREFUL NOT TO DROP THESE SCREWS INTO THE AIR VENTS.
b) The cluster is a bit of a pain to get out if you have large hands. There are two screws (which will have red paint on and in them, if your cluster is stock and never messed with) on the upper corners, and you will need to carefully and gently pull the top edge of the cluster forward to you while pushing the bottom away. This will let you reach over and behind it to the two wiring harness plugs into the back of it. The one closer to the passenger side has a single wire so be careful to not pull by the wire - use the tab on the harness to pull the plug out. Once those are removed, you should be able to get the cluster itself out of the dash.
6) Final preparation and then removing the dash shell itself
a) You will need to remove the lower interior panels next, I would suggest starting with the one which holds the door to the fusebox on the driver's side. It has a view easily found and worked Phillips screws... and then the most annoying fastener of this whole project:
This last screw is holding the rightmost edge of this piece onto the center column itself, and it is within a molded niche of the plastic - designed to leave you "plenty of room to use a screwdriver" I thought, it won't work. The above photo shows the local hardware store's smallest ratcheting screwdriver and that was still slightly unwieldy in this small space. Be patient and get the screw out, don't tug too hard on this piece until you're sure you have all the screws out.
b) The two metal triangular plates hold the weight of the dash shell up can now come out, I believe they have three 8mm hex bolts each. I am tall and held the dash shell up with my knees as I did this - but you might want something sturdy to prop it up as you remove them. Even with all the internal parts removed, the shell is heavy enough to fall hard.
c) The knee bar is one of the last pieces to go. It has two bolts on each side (each pair under a shared plastic cover), and a forewarning - this is a part of the project best done with two people. Loosening your side and holding it up for the other person to do the same - otherwise, it is likely 2 of your bolts will go FLYING into the unknown recesses of your garage.
d) I didn't take a good photo of this next step because I didn't realize I forgot it until it was too late: removing the two clips which hold the central HVAC duct built into the dash shell, to the front of the air box. This photo points out with green circles and red arrows where the two clips are located BEHIND the dash shell:
This is what the two connectors actually look like, once you remove the clips from them:
I did actually break both of these because I didn't see them the first time I tried to very gently remove the dash:
That said, I was able to rebuild the entire dash without gluing the broken tabs into place (so if you bust them too, don't worry too much). But for this wiki guide, I am suggesting: remove these two clips right the first time.
**There is also a single Phillips screw on the bottom front of the air box holding the central duct to it.**
e) There are two bolts on each side of the dash, underneath circular plastic covers, holding the shell into place. Remove them carefully. The dash shell will likely be loose at this point,but still has HVAC ducts connected to it, so don't pull too hard. If you sneak a peak underneath of it, you'll have a look at the elusive heat/air box itself, and the problem foam (or whatever scraps remain of it):
DO NOT TRY TO YANK THE DASH SHELL OUT YET. You need to deal with an odd design choice by Volkswagen when it comes to wiring harnesses - namely, this:
That's right, they decided to stow some of the wiring within a triangular plastic internal support for the dash shell, held together with a rivet. Being careful to hold the wiring far away, drill out the rivet with increasingly larger drill bits until it falls out:
Drill the hole big enough to fit a machine screw and nut but not so big as to compromise the structural integrity of the beam. Then be ready to have to do a mixture of brute forcing the support open to allow the wiring to pass through it, but gently enough to avoid breaking it.
f) One of the most annoying parts of this is next - a combination of elbow grease, patience, and creative swearing will be required to remove the HVAC ducts from the dash shell. The ducts are all held on by molded plastic integral to the dash shell which has little tabs or bumps as part of their design - great to prevent the ducts from falling out... which means it is a horrible pain to remove when you want to do so.
The central duct is part of the dash shell, it has a duct go to the passenger and driver side - each of which is removable once you dislodge them from their mount points. I suggest doing this to avoid dragging any wiring harness with you unwittingly.
The dash shell will be easily moved partway off the under-dash at this point:
g) Now, go around and unplug the remaining wire harnesses to the components integral to the dash shell - the two speakers, a metal component on the leftmost side of it with two plugs. The car's wiring harnesses will mostly be hooked into the inside of the dash shell, using plastic wire holders - you will need to be sure to remove all of these.
h) Now, gently pull the dash shell straight towards the back of the car. I suggest being very cautious and willing to stop pulling if you feel any serious resistance... it is possible you forgot to remove or unhook something. Patience is key here.
You might need to lift it slightly and then pull it back, to unseat it from the two metal guides on the under-dash. I suggest two people for this part of the project, to avoid banging the shell too hard or scratching the car as you try to remove it from the cabin.
You now have the entire dash removed, congratulations!! Gaze on your work with pride, and go take a break:
7) Dislodging the heat/air box (people refer to it both ways).
a) The first step here will actually take you into the engine bay. There are two notches in the firewall near the coolant tubes, each has a 12mm bolt holding the heater core and heat box into place - loosen them fully and remove them, and keep them safe somewhere.
b) I guess it is possible to do this job without draining all the coolant, but you will spill some if you don't. However you decide to do it: take the two coolant tubes off the heater core pipes by loosening the spring clamps.
c) Now comes another one of the core annoying aspects of this job - removing the heat box by taking off the 5 or 6 clips which hold it in place. I had a set of various Craftsman manipulation tools, I found the L-shaped one (when paired with the screwdriver magnetizer) most helpful:
The clips each have one side which is only curved, and then the other is curved and at its base is a flat tab protruding out almost perpendicularly - this tab is how you will actually remove most of the clips. They are brass colored, and you will see approximately 3 of them easily - two others are along the right (passenger) side and can be felt if not seen.
I say "most" above because the last clip is not reachable by human hands, it seems - it is at the bottom and the back, and people have only been able to remove it at the very end by gently twisting the heat box back and forth in place.
d) Please realize: the heat through these ducts will make the plastic sit SNUGLY in place, so you will need to be on the overpowering side of gentle when trying to pull the heat box straight out of its place. You might need to pull at an angle first, towards the driver's door, to unseat it from the duct to its right (towards the blower motor etc).
Removing the old heater core/installing the new one into the air box looks the same:
There is a plastic bezel which holds it into place, with two Phillips screws you must remove. Once you take them out, you can lift the core out (be wary of spilling coolant into your car, have rags ready). Now, the heat box and the cable-connected HVAC controller will lift out of the car easily.
You've nearly made it to the halfway point of this project!! Take a break.
8) Rebuilding the heat box and the HVAC foam on the blend door, mode door, and defrost mode door.
*NOTE*: this is the central task for this whole job, rebuilding the air box. If there is only one part of this project you're going to spend extra time to make sure you do it right, this ought to be it.
As per all steps of this project: LABEL EVERYTHING *BEFORE* DISASSEMBLY
Air box rebuild 1) Be sure to know that this will be a messy process - between disintegrating bits of stock HVAC foam and dust, you want to not work on a nice Thanksgiving tablecloth (like I did at first):
Air box rebuild 2) Disassembling the air box is a bit nerve-wracking. This is a close shot of one of the clips, you need to be in a warm place (so the cold doesn't snap the plastic) and gently remove one of the tabs through its catch. From here, gently pry the box halves apart starting at the next closest clip - do not be afraid that it will likely pop apart in a loud, sudden, and unexpected way - but it won't be broken.
Air box rebuild 3) Looking down at the top of the air box you will see the defrost mode door - a vital component you won't be able to see UNLESS you disassemble the entire dash and remove the air box. Its stock grey foam gasket ensures all the heat possible reaches the defroster vent, and so you'll want and need to take it apart for rebuilding, too.
Air box rebuild 4) The most important tool for this job will be Goo Gone, as you want to get the parts extremely clean before applying new adhesive foam. Be sure to follow the label's instructions: using dish soap to clean the Goo Gone off after use, and then rinsing with clean water before drying the surface.
Air box rebuild 5) After cleaning the blend door (the door with the angle in it, this corresponds to the Hot-to-Cold dial on the controller, and will decide how much cold air [either from the outside air or the A/C ] and how much hot air from the heater core to produce differing temperatures output air flow) and the mode door (aims the air flow either to the front upper vents or the lower vents, OR the defroster) fully, size them out on the McMaster foam, and then carefully cut to size using the Fiskars cutter.
Note that the small black plastic cylinders to allow the doors to turn will have the ancient crumbling remains of the stock grease on them, and can easily be removed for cleaning.
Air box rebuild 6) After applying the foam to one side of the door, I strongly suggest using some of the HVAC tape to cover the other side of the doors. This is both to ensure a better seal for the air flow, but perhaps more importantly is designed to help insure that the adhesive backed foam will stay - that adhesive will one day go bad, even though it is premium McMaster stuff, and by wrapping the tape over the edges of the door onto the foam, you are guaranteeing it will last that much longer.
Air box rebuild 7) As you begin to put the pieces together, be sure to use a toothpick to carefully apply a modest layer of the Lucas grease to all points of movement for the 3 doors - the defrost mode door shown before has two plastic nubs on it which are its axis for movement; the two larger metal blend and mode doors use those small clip-on black plastic cylinders to enable movement. In both cases, you need to clean off whatever small amount of the stock grease remains, and apply this stuff. This insures the doors will work smoothly and quietly for many years to come.
Air box rebuild 8) This is how the box should look right before you put it back together - test fit and test spin the doors in all three places. Be especially sure to try the HVAC dials on the controller, to make sure that the defrost mode door works properly. It IS possible to make that foam gasket too large, in such a way as to inhibit proper movement... so test everything out along the way before closing it back up.
Air box rebuild 9) the last step is getting the whole assembly back to the car, and reinstalling it. Know that you won't be able to reinstall that final clip on the bottom/back, but all the people I have spoken to who have done this report no issues with HVAC for thousands of miles after the fact.
A brief break in the instructions here, as I was bleary-eyed after the air box rebuild when I did this - I suggest you take a break as well.
9a) Reinstalling the heater core and heat box
a) Prior to trying to reinstall the heat box, you should remove the single screw holding in the upward facing shunt (see the photo). This will allow you to use Goo Gone and remove the rotten foam gasket. After soap, rinse, and drying, I used some of the remaining McMaster foam to rebuild the gasket:
b) I suggest the above break mostly because whereas getting the new heater core into the rebuilt air box is easy... get the air box back into place in the car is a huge pain. In my case, the all-metal Passat B5 heater core tubes have a SLIGHTLY different and higher angle to them, as you can see here:
As such, the B5 heater core doesn't work with the stock coolant flange on the firewall (I spent nearly 3 hours fighting with it, with another person helping, so don't waste your time).
If you chose to replace the heater core with the stock plastic tube style, you will have no problem - just remember to replace the silicone vacuum tube from the flange to the blower motor inside the cabin...
...and be VERY careful to not break the white plastic nipple on the flange (the below image is the B5 core, but I wanted to illustrate how it would look using the stock plastic coolant flange):
To be certain you have a good seal between the heat/air box and the duct to its right (the passenger side) which goes to the blower motor, you will hear a loud click or snap as the box seats properly. ONLY after that should you get out and tighten those two 12mm nuts onto the threaded rods coming through the firewall into the engine bay.
c) Before you go any further with this project, you need to test the HVAC controller knobs and then lubricate them. I suggest doing this after the box is reinstalled, but before any additional work - you want to make sure it works in place, but without having to remove anything else if they don't work for some reason:
Once you're sure all three HVAC doors are moving freely and properly, use some of the Tri-Flow lubricant you bought on both ends of each of the HVAC control cables. Vacuum clean the interior of the HVAC controller (mine was covered in rotten foam dust) and then use a toothpick to apply Lucas #2 grease to the gears that the knobs control. A tip for not applying too much - apply a little, turn the knob fully both ways, then apply a little more. You want an even coating which will last a long time... and not a mess.
9b) Optional coolant flange fabrication
NOTE: the following only applies if you choose to use the all-metal Passat B5 heater core as I did. It involves fabricating a new flange for the firewall where the heater core coolant tubes go through.
Coolant flange 1) Use the stock foam gasket to shape out the aluminum shim material. You want to cut an internal notch JUST large enough to fit around the tubes, but otherwise as snug as possible.
Coolant flange 2) After test-fitting the aluminum shim and bending it as best you can, use some of your remaining McMaster foam to cut a new gasket the size of the shim, and cut careful holes for the tubes. Make the holes a bit too small as the foam can flex, and you want as strong a seal as possible. Be sure to leave a lower notch (see #3 for explanation)
Coolant flange 3) Use some more of the extreme weather aluminum HVAC tape to cover the shim+foam gasket. This will help both keep the foam in place, and increase its heat deflection properties. The lower notch is vital, to allow the vacuum line to the blower motor inside the cabin to run through, and you don't want that connection too tight (to avoid putting a pinhole into the line).
Coolant flange 4) Using high temperature RTV, make the best gasket you can around the edges of the shim to the firewall. Note that the aluminumn is super thin and easy to bend... but will be difficult to bend to the EXACT uneven topography of the firewall. My hard-earned wisdom on this: bend the aluminum to size, take a 10 minute break, come back and realize "it doesn't really fit, let me fix that." Repeat this a few times until it ACTUALLY sits flush to the firewall.
Coolant flange 5) Finally, use at least two additional layers of the HVAC tape to get the shim firmly sealed down. My experience is that this tape sticks best to aluminum... and the firewall is covered with a dimpled aluminum, so I believe this will hold for a long time to come.
10) Final considerations and helpful tips for rebuilding or repairing as you begin to reinstall
10a) Rebuilding and cleaning the dash shell
a) You're going to want to clean the dash shell vigorously BEFORE even putting it back into the cabin of the car. It is very dusty and dirty, and will have a lot of bits of rotten foam on and in it. If you have the thing out, do this right:
You will also see the rotten skeleton of foam on its two sides - to prevent vibration against the pillars:
I replaced this with a McMaster foam sheet I cut into strips - and then I noticed the rotten foam along the bottom edge of the windshield:
It takes a while to cut the super cushioning foam into straight lines, but this will ensure both a good seal for air within the dash shell, and guarantee no rattling of the dash against the windshield for years to come:
You ought to use some of this same foam to replace the destroyed foam gasket on the passenger side vent nearest the door, after the usual Goo Gone cleansing:
... and while we're on the topic, the rear of the central HVAC duct had a foam gasket, which needs replacing too:
10b) being absolutely certain the HVAC ducts are properly seated before bolting the dash shell back in
This seems obvious, but once you start to wrestle the dash shell back into place, you might have this happen to you:
It is hard to see, but that is the point - it is hard to notice this if you aren't careful. That new orange gasket is NOT mated up to the bottom of the central HVAC duct in the dash shell - which means something isn't seating properly. It is possible to reinstall the whole dash without realizing this - and thus compromising the air flow you worked so hard to repair. Be absolutely sure that this central connection (and indeed, all other HVAC connections) was done right and is fully seated.
10c) Making sure you keep all the wiring harnesses straight
I realize some people are still nervous about wiring. I took photos of each set of connectors you need to remember to reinstall, and these are in order from leftmost (driver's side) to rightmost:
Wiring set 1: 2 into metal component within dash shell; 2 go to driver's side speaker; white and blue go into back of instrument cluster; large black goes into back of headlight switch
Wiring set 2: Steering wheel harnesses. Two go into left stalk; three go into right stalk (including the red one). Yellow connects to the rear of the steering wheel's internal airbag sensor line
Wiring set 3:the OBDII port (which is a royal pain to remove from its bezel, for some reason). Don't put this back until you're 100% sure you're ready to move on.
Wiring set 4: Center console wiring. Radio and antenna; ABS and airbag lights; rear window controls; cigarette light power.
Wiring set 5: These are the two outliers by the passenger door. The pair in front go to the passenger side speaker; the two wires in the middle of the below frame are (accidentally) pulled out of the yellow plastic plug which goes to the passenger airbag:
Reinstallation is approximately the reverse of the above (I offer multiple rebuilding tips interspersed with the disassembly instructions). Note that it is well worth double- and triple-checking each step along the way, to ensure you get all the components back into place rightly before trying to add something else back into the dash.
Many thanks to all those on the TDIClub forums who offered me advice along the way on this.
myturbodiesel.com has been a great resource for me as I have learned to keep my car going strong (257,000 miles and counting), and so I hope this guide helps others who are new enthusiasts to this B4 hobby/ownership like me.