I don't remember where all the specific grounds are on the mid 80's Vettes, but I do recall that a bad dash ground was common. The simpilest cure was to run a wire from the dash grounds (I believe there was 3 separate ones) directly to the chassis. As for the mirror, sounds like a bad switch. At the cars age, it is past due the time to clean and tighten all grounds, replace broken, frayed, or hardened ground wires, and spend a bunch of time doing resistance checks and voltage loss checks on every electrical circuit in the car.... Ain't fiberglass wunnerful????