Just had my wing mirror apart, as they weren't working.
testing the voltage i found that they heated separate from the rear heated window (as has been pointed out here) but as i have a chrome switch from an online auction site, it has more positions than my old one.
It only heats my mirrors in the "duel mirror control" position, and does nothing in the heated position...
Also doesn't help that there is little to no docs on fuses for my 2004 AXX GTI, but at least i don't have another fault.