I can't see how a mirror wire would have anything to do with the brights not locking on.
As for that mirror, you said "rearview...", I'm assuming that means rearview, not sideview. There may be an extra connector tucked up in the headlilner as part of the wireing harness where an optional accessory wasn't factory installed. For example, my 1996 had an optional telephone in the armrest, the connector for the microphone is sitting above the rearview mirror, connected to nothing.
Dimming mirror, Map lights, Compass Module, and possibly some other things (depending on the year) I can't remember at the moment all have wires above the rearview mirror, none of those have anything to do with highbeams controls circuits.
There's also the remote posibility that the rearview mirror currently installed wasn't the one the factory installed, and some option is missing.
Damn I wish I had the 89 manual...
Did you try different column tilt positions to see if there was any difference ???