Only the 2005.5 VW Jetta and 2012+ Passat came with an auto dimming mirror. It clips onto the rain/light sensor so the mount is different vs. non-dimming mount. You could add the rain/light sensor but you must have a highline control module and you have to add the wiring to make it work. See 1000q: auto wiper-headlight and dimming mirror for how to add the OEM parts.
The reason to go aftermarket is because they cost about 8 times less than new
OEM parts. I bought my OEM mirror in a junkyard but again, to add the OEM VW mirror you also have to add the OEM mount. Late model Audi A4 mirrors without the rain-light sensor are direct bolt on but you have to add your own crimps and wiring. Make sure you don't get an Audi mirror with the rain-light sensor base because it won't fit your existing non dimming VW mount. Below are some examples of Audi A4 mirrors that will fit. They come with a compass but not homelink (click to enlarge the image). Some may look slightly different.
Gentex is the supplier who makes the OEM VW/Audi dimming rear view mirrors so you could buy an aftermarket Gentex part and use that instead and the aftermarket Gentex part can come with homelink or a compass. Some Gentex mirrors are also slightly larger than the OEM mirror. The Gentex mirror base is also more square whereas the VW mirror base is a circle.
Mk6 VW Passat and T2 Touareg mirrors have auto dim but they also have a different part number than the Jetta mirror but use the same windshield mount. If you use a Gentex mirror you have to use a mount adapter between the Gentex mirror and the OEM mount. Using the adapter is much easier without the risk of damage to the windshield when removing the original mount.
Pics mirrored with permission from TDIinMA, veraciously, and av8r, many thanks! This page summarizes info from many sources into 1 short page.
If using a Gentex mirror you need this adapter: http://www.parkingsensoronline.co.uk/adapter-gentex-mirror-fitting-p-287.html
1 auto dim mirror
T20 torx screwdriver to remove the map console area
Grab the base and turn it clockwise 90 degrees. Don't pull it too much
because it's possible to crack the glass. It does not screw off.
it 90 degrees and then wiggle it off. Below are pics showing the OEM
mirror base and windshield mount. Turning it unlocks the base. The
adapter required to fit the Gentex mirror to the OEM windshield mount is shown
to the right and can be purchased from the above link. You can paint it black to make it less visible. Because
it may not fit perfectly, try putting some thick foam or a few layers of duct
tape around the adapter to make it fit tighter. Not shimming it can lead
to rattles due to a tiny amount of play.
The mirror requires power and ground to work. You could run a wire down to the fusebox but it's easier to tap into the overhead lighting with the supplied crimps. Remove the front of the overhead console by prying the grille off and then removing the T20 torx screw. Caution - because there may be differences between years and models so always confirm your wiring with a multimeter and the factory service manual's wiring diagrams!
Remove the overhead panel by pulling the 2 piece cover off and removing the 2
The red wire with blue stripe on the map light harness is switched power
(12V) and the brown with purple stripe (2nd in from the end) may be the constant
12V. After the ignition has been off for a while, the switched power goes
to 0. This is why the map lights go off by themselves if you leave them on
with the car off. This will also disable the homelink when the power goes
away. Other people found a black wire with green stripe was switched
power. 2010 or newer may use a green wire w/yellow stripe for switched
power. Use the crimps to splice into the wiring once you have confirmed that this
is correct on your car. It doesn't matter if you have a sunroof or not
because you want to find the overhead lighting wiring, not the sunroof wiring.
Here is another shot from av8r
Bad grounds can cause a number of problems. Find a bare metal bolt on the chassis and clean the metal around the bolt hole with some sandpaper. It must be bare metal. The ground wire will go there. If you can't find a suitable ground hole, drill your own hole.
Many people complained of ratting because the mount adapter has a little play. Put a few layers of duct tape on it to act like a flexible pad for a tight fit.
Slip the new mirror on and test the function. The mirror shown below
has homelink function in addition to auto dim. From outside the car you can see the square bottom of
the mirror mount vs. the round mirror base. It's difference between the
OEM mount base and the aftermarket mount is small enough so that nobody would
notice it if they weren't looking for it.
Wider picture showing the size of the mirror in context.
If you have more questions about installing an auto dimming mirror, please post a follow up question in the VW TDI forums or search the site below: