1999 Mitsubishi Galant rear defrost

Probably the fastest way to start the diagnosis is by bypassing the defogger relay to see if the mirrors and rear window get warm. The relay is inside the car on the left side of the dash. I don't know what the terminal layout looks like, so the easiest is to pop the cover off the relay, install it that way, then squeeze and hold the movable contact. If you see a tiny spark in those contacts when you release them, that's a good clue that part of the circuit is working. If it is, suspect the heater control unit, but there are a few more tests we should do to rule out other possible causes.