Well, I found that the mirror's turn signal really is bad. Starting a separate thread because it's a separate topic that will have diverging follow up posts. I swapped it with the turn signal from the Passat and the good one worked on both cars, the bad one worked on none. I think 1 LED failed or a solder broke because 3 LED still light up dimly. The unit is sealed so there are no bulbs to replace. The mk4 Passat B5.5 uses the same side view mirrors. I noticed a few differences vs. the mk5 Jetta parts. The wire to the turn signal doesn't have a keeper on the Jetta. The passat has a little peg/hole that holds the wire. The Passat uses phillips screws, the Jetta uses T10 torx screws. The OEM part was cheap enough so that I just ordered a new one online. They sell copycat parts on ebay but it wasn't worth it to buy a pair or buy one and end up with one that side that looks different from the other.