I learned another something new. The outside remote mirrors in '88+ models are slightly different than the '87 and below models. The former have a second black and white wire connector (two prong) on both the drivers and passengers side. Maybe it is some kind of ground?
The latter does not have the second two prong connector. The drivers side has a single black and white wire that is taped back onto the wiring harness but not connected to anything. The passenger side does not have this wire.

 

Heated mirror wiring, I suspect. The heaters work when the rear defogger is on.

Another 'new feature' for 1988 MY was the heated screen washer jets. There's a temp sensor behind the radiator grille.

Cheers
DD

 

Thanks. So this should mean that I can replace a mirror that either doesn't work or vibrates badly with one from any model year and only need to be concerned about hooking up the four pin connection.

 

Correct. Just disconnect the heater wires at the plug inside the door. Also, if you have the tiny little white mirror electrical connectors, clean they very carefully, and emery paper the prongs too. 90% of the time a non-working mirror is poor conductivity in one of those connectors. Also, using a small cable tie to hold the connector together is a good plan.
Greg