There were some A4/A6 cars out in the yard. I removed one and it used the same windshield mount, but it had plastic trim that extended to the headliner with wires hidden under the plastic trim. I wasn't sure what to do with all that stuff, if I used the mirror in my wagon, so I just left it all on the dash for someone else to retrieve. Is that the same kind as what you're talking about? It had a little window on the lower right part of the mirror glass. Not sure what that was for.
Here's the two-tone MK5 mirror that I installed in my MK4 wagon:
I also have the all-black Passat mirror, that I purchased last week from the yard. I found the two-tone and bought it because it was in great shape and thought it would be fun to have options, since it's so easy to swap them out. Plus, they're ridiculously affordable at the Pick-n-Pull.
The little trim is where the wiring for the auto dim/compass is hidden. The "fit" should be pretty close, if not, you just trim the end where it would touch the head liner.
As for wiring, the simple is to tap into a switched 12v and ground, the 3rd wire would be to cancel the auto dimming when the car is put into reverse.
The mirror side of the connector will most likely have 6 wires (maybe 5), the car side will depend on the car it's from and what features/options it has.
I'd suggest popping the dome/sunroof controls out of which ever car you're getting the mirror from to gain access to as much "length" of wire on the car side of the car side connector.
The black "window" on the auto dim mirror is for the normal compass. If you can find a A4/B6 with the wire cover/trim (indicating auto dim) and no black window, it is most likely a "hidden compass". The compass designation appears in about the same place, but seems to "float", vs having the designated window.
PM me if you're interested in building your own auto dim, and I'll answer any questions you have.
If nothing else, grab the hidden compass (w/ car side connector/wires) and sell them.