I noticed this today with my car. Dew on the mirrors and no heating.
Went to the dealer during my lunch and discovered that it is indeed temperature dependent and according to the outside temperature - they will work at 100 75 or 25 percent of full power !!.
To activate the heaters when the temperature would normally prohibit them, simply turn on the wipers and this turns the mirrors on - but if its not too cold - it will still only use part power. You only get full power if it is below 10C.
Another example of how too much complication and technology can ruin something that used to work so well - just like the indicators !! Wake up BMW and stop applying technology just for the sake of it. Often it doesn't work that well