on my 1990 944 convertible, my mirror are heating all the time making the glass loose cooking the glue...what is the switch since I dont have a rear window defroster (neither a switch) !?

 

You know, i have never looked into why that system works/when it works. Might want to pull the mirror glass off (there is a plastic ring you have to rotate on the backside of the glass through a small hole in the bottom of the housing), and check the wires. Usually the wires are in pretty rough shape.