firebobby said:
Could be the switch, it's separate to the mirror control. It may be worth trying an electrical spray cleaner on it. I had this on an old 5 series that my mum had, a couple of the windows didn't work from the main switch, so plied the buttons with WD40 and after a few button movements they came back to life
If condensation forms on the cold drink you have in the cup holder, then it can drip down on the buttons.
I have thought that this could cause damage... here we might have the answer?
Try some electrical cleaner.