Notes before we begin:
If you have a W123 with loose mirror mount (stalk to vehicle link), then chances are one or more of the 3 screws holding the base is loose.
There is a way to reinstall the mirror pretty easily, and removing it is as simple as yanking it sideways.
If the mount is loose, it will give you difficulty in removing, because the base is yielding to your pull, preventing the stalk to separate from the base.
Couple of notes from the installation:
The installation went smoothly. Instead of removing the A pillar, I just slipped the extension wires through the sun visor mount holes.
1. Download the auto dimming installation guide
from Rik Johnson's site, to get an idea on how to proceed.
2. Remove the mirror by yanking it sideways. GRAB THE STALK, not the mirror.
3. Remove the dome light and electric connections.
4. Remove the sun visor and clip on side you want to run the wires through.
5. Pull out the weatherstrip material that surrounds the door opening. This material is the type that starts from the top of the B pillar, forward to the A pillar, and down to the front portion of the door sill. No need to remove it completely, just out of the way where the dashboard, A pillar and weather strip material meet.
6. Slip a 20 or 22 gauge wire into the sun visor mount opening (not the clip opening) until you reach it from the dome light opening.
7. Slip the other end of the wire into the sun visor mount opening, until you can pull down on the header material and slip the wire out. Run the wire on the rear edge of the A pillar trim, and using a flat blade screwdriver, CAREFULLY push the wires into the space between the A pillar frame and A pillar cover.
8. Slip the end of the wire into the dashboard. This is to be accessed via the glove box opening.
9. Tap any SWITCHED 12V source you wish. In my case I used the glove box door, but note that if you use the glove box power leads, make sure you access the leads closer to the insulation material. This will assure that you will tap the ground wire that is NOT deactivated by the glove box door. Otherwise, the mirror will run only when the glove box is open (and vehicle is running).
10. Mount the mirror and test. If it tests good, then you can reinstall everything back. If not, determine source of problem and troubleshoot.