Just got two brand new mirrors from Audi to replace both the passenger side (no longer heating) and the drivers side (heating, but has a rusty mess under the glass).
I fitted them and checked that they were heating up when the ignition is on (my pre-facelift S3 permanently supplys 12V to the mirrors when the ignition is on) - they worked fine. Then I took a short trip to the shops and when I got back I checked them again and they were stone cold!
At first I thought maybe they only operate during the first few minutes of switching on, but now I've checked the fuse, and had the mirrors off again to find amazingly that both mirror elements have failed. I've checked this with a multi-meter, the old working mirror has ~20ohms across the contacts where the new mirrors are open circuit. Sure enough re-connected the old mirror and it works fine.
Needless to say the ones I bought will be going back, but surely this is likely to happen again with any replacements. Is there anything I could have done differently to avoid this?
Oh, and I did check the part numbers and the old and new match exactly. Reckon I'll be "running in" any new ones with a 12V power supply for a while before I bother to fit them whether that will make any difference.