I have done this on other cars.
There is only a foil type material with a wire in it that heats up, if you find any that will just fit the back of the mirror, all you do is buy it, peal and stick, run 12V to the wires and install a relay, switch and proper size fuse and you are good to go. However, you can burn em up if you leave em on when it is too warm out, so consider that. Some have thermostatic control where they don't turn on unless it is cool enough and some have timers but most are just wired right into an Ing. Source switch only.
Only point, don't buy one too big and try to cut it down, once you jack with the wire, it won't work. Smaller is better, mount it low and it will work fine when the heat rises. MC