I got some questions about those heated mirrors I have. Now here's how it's done.
I used a kit from the UK that came readily with an switch and some wiring. The heat elements are simple electric resistors that should be available at any electronic shop, they are marked 17 Watts each.
To remove the mirror from its housing just peel off the surrounding rubber. This must be done on a warm day or with the help of a heat gun.
Clean the back with some window cleaner and let it dry thoroughly. Apply the heat elements and let them sit for some time. Now you must drill a small hole in the bottom of the mirror housing so to pass the wiring. Use tie wraps in black to secure the wires to the arm.
Peel off the rubber door seal near the upper hinge. Drill a hole or cut the metal lip so you can pass the wiring through. You only have to install a switch at a convenient location. I also added a control light by using one of the unused indicator lights in the warning light cluster. You won't need a relais as the elements only take 35 Watts but make sure to install a 5 Amp fuse.
A friend suggested to use a light bulb inside the mirror housing. It will surely work but I think as the heat is focused in a small place it will burn the mirror after some use. So you will have to protect the back with a small piece of metal.