I know theres an heater element in them that warms them up. I'm more concerned with how they operate.
In my 80 quattro, the mirrors would warm up when I turned the rear defroster on. Not the case with my 96 a4. Do they work by temperature? I don't see any other switch that would activate them.
Now I've read about the "browning" which is caused by the heating element expiring. Would that cause the element to stop working? Both my mirrors have a bit of browning around the edges, which doesn't bother me much, but if that's causing them not to warm up, then I'll be bothered.