I have a 74 plate A3 sport which is currently at the dealers been for two days with no heated mirrors. My partners car is the same age and model, I had to take it in to the dealership to show that even at 15 degrees outside her mirrors heated up fine. This is my fifth of this shape all except this one the mirrors heat up, mine have not worked since delivery, and it is so dangerous on misty damp mornings. I have to clean when I leave for work and wipe again part way through on a 15 minute journey. I know that my car has also had a technical upgrade to the software to sort a plethora of issues on the mmi. If it is set to 5 degrees I guarantee I will be speaking to customer services pointing out it is dangerous. From what Sam said above it would appear some are fitted with different control modules and mine is one so probably without admitting it Audi have realised they have messed up again trying to be too smart.