yes temp dependent. I have the exact same question for my 08 X3.. the glass is ICE COLD. Does not defrost anything.

Loooking at the manual it states they are controlled by outside temperature and its not a special feature as in its standard option.

Not sure why they don't work. Im gonna remove the glass and look at the back of it to see;

I did find this, but its for a E36.. http://www.bimmerforums.com/forum/sh...heated-mirrors