I'm going from memory here because I have the operational specs of how the heating is activated and the sheet is in the barn and there is a foot of snow on the ground, so I don't feel like walking over there and getting it. I had a Tech print them out for me a few years ago.

Anyway, there is not a manual way to activate the heating function. The heating elements are controlled by computer through the car's CANBUS. The temperature that they activate is at or below 48 deg F. if I remeber correctly. Also the wipers have to activated and cycled for the heating to activate. I think the car assumes that the mirrors will fog in high humidity (along with the windshield) and that the driver has the automatic wipers activated, or has recently done a manual swipe of the wipers.