The rearview mirror in my 1984 300D self-destructed. I wasn't even flipping the high-beam shade, just barely touched it and the bottom fell out. I expect cars in other "evil sun" locations will eventually experience similar.
The best link I found:
www.peachparts.com/shopforum/diesel-discussion/368781-mirror-removal-technique.html
Would have been smart to search that before I went to PickNPull. I couldn't find a hidden cover to get at the mirror post screws on a W123. They don't exist. The post is secured by two spring-loaded side cones. You tilt it sideways to pop it out of the bracket's rectangular hole (bracket bolts to roof thru head-liner). At PickNPull, I had no insight, so pried the mirror off the post sphere. That broke some of the mirror's securing tabs (old & brittle), so I looked for another mirror that would work, and learned about the various designs. A Jag mirror looked like it would work except electronic and the wires route thru the sphere. Finally, I found a W124 (recall 94 300D) that looked identical so pried its mirror off the post w/ no harm.
While trying to pop the W124 mirror on at home, the post in my car broke free and I then saw how it is secured. The W124 mirror was too tight to pop on the sphere by hand, so I used my shop press. The mirrors look almost identical, but PN's differ:
1984 300D: IE1 0010 059, Ie102*10059
W124 (1994?): IE1 0110 050, Ie102*10050
The only difference I noted is that the later mirror has a small rubber bumper on each side, perhaps for windshield contact. Otherwise, the advantage is that a W124 mirror is newer and hopefully less brittle. In the distant future when none are available, you could remove the post and use a "glue to windshield" type, though my 1996 Voyager has one and it has fallen off 3 times so far (why auto parts sell the super-glue).