Easy as pie.
Remove the two screws underneath the courtesy lamps (have to carefully pry the lenses off first), removed the two screws on the outsides of the console out of the visor clips, then pulled down the entire overhead console.
After that, reach in and to your right, you'll find a wire attached to the rest of the harness... it's taped up pretty good though, so carefully undo some of that tape, and you'll cut loose the harness for the mirror. It's complete with plug, and even the protective nylon jacket!
I put my headliner back up and re-installed my console, then simply removed the original mirror and installed the auto-dimming mirror.
Piece of cake, and an instant upgrade! I used the mirror I had left over from my 90 Imperial, which used the same mirror as my 91 would have came with from the factory, had it came with the auto dimming option.
Just thought I'd share!