2 wires....power & ground....done

Ya, but I have to find one first!

Then, how do you calibrate the compass and thermometer? Aren't there more wires involved than just those 2?

Ugh... Sorry. I didn't get it the first time. It was late.

As I said before, mine took THREE wires (ground, constant power, ignition switched power). If you get one with the temperature, you'll also have some kind of lead for the temperature probe (usually located near the front bumper). The complexity of the wiring will increase with the complexity of the mirror. Even the most complicated mirror won't be that hard though, especially if you can steal at least some of the original harness.

Compass calibration is quite simple. Mine did it all by itself in a matter of about 2 miles of normal driving, but I'm told that if that doesn't work, go to a parking lot and turn left in a circle for a while. As soon as it displays actual directions, you're set (when it needs to be calibrated, it reads "C"). As for calibrating the thermometer, I don't know. Does it even need to be?

I find myself wondering if mine is the first '87-'91 to be equipped with one of these things? I somehow doubt it, but that would be cool.