I've left a lot of the details out of this how to, because I can't really see anyone else doing it in full, so if you have any questions just post a reply.
The rain sensor is hit and miss but the mirror is brilliant for night driving. You can install the mirror on its own without the rain sensor.
The rain sensor instructions will apply to any car with a 19 or 99 wiper relay, e.g. Mk2-3 Golf, Mk2-5 Polo.
Read this first - it's about doing this mod to a Mk4 Golf. It doesn't cover auto-dimming door mirrors - as far as I know I'm the first to retrofit them to a VW. And I'm not responsible if you blow yourself, your car or your dog up.
Parts Required:
Wires and connectors - see pdf
Araldite Clear Glue
For autodimming interior mirror only:
Mirror 3B0 857 511 H
Channel 3B0 857 591 A
Channel 3B0 857 594 A
Outer trim 3B0 858 547 A
Outer trim 3B0 848 548 A
Conduit 3B0 857 594 A
Cap 3B0 858 548 A
Mirror mounting 3B0 845 543 A
These are easy to get from a breaker.
If you want autodimming door mirrors too:
Mirror glass from an Audi A3/A4/A6/A8 around 1998-2003. There are loads of variations.
4B1 857 536 L is a perfect fit in a RHD stubby driver side mirror.
You can also fit huge Golf Mk4 mirrors and use these mirrors:
4B0 857 536 A
Here's the socket for the autodimming wires:
4B1 857 535
And if you want rain sensing too:
Rain Sensor 1J0 971 411 A
Relay 6N0 955 531
Mounting the mirror and rain sensor:
First remove the old mirror. Turn the base of the mirror anti-clockwise about 45 degrees. It's very easy to break your windscreen doing this, so maybe use a cloth to stop yourself putting too much force on it.
If you haven't already done so, smash your windscreen now. Replace it with one with an autodimming mirror mounting. Stick the rainsensor within the mirror mounting.
Or just do what I did and glue the new mirror mount around the old one, and glue the rainsensor to the glass just above the mirror mount.
Fitting the auto-dimming door mirrors
The Audi mirror glass just snaps into place exactly the same as the original glass.
Wiring:
Altered from BazMcC's diagram on uk-mkivs:
Pictures:
The interior mirror off and on. It was too hard to take night time pics of this.
The autodimming side mirror in action in daylight:
And at night. You get the idea...
hats off to you