We have a 1993 W124 Chassis, 300D 2.5 I've got a new driver's door window regulator for it. (The old one has torn up teeth).

When I removed the old regulator, I found a broken plastic disk/washer, about the diameter of a quarter, inside the door. The new regulator came with a new plastic disk piece attached to it with a twist tie. I don't know where this disk is supposed to go. My best guess is that it is supposed to be stop to give the regulator a "soft" stop when rolling down the window. But it is only a guess.

I've looked a various parts catalogs, including epc.startekinfo.com, but I've not found an diagram which shows this disk. I'd appreciate it if somebody on the forum can tell me, it will save me dismantling the passenger side to see where the part is supposed to be installed.

You are right, it goes on one of the posts on the motor that the bolts that hold the regulator on screw into. Like you I struggled in figuring on which one; the one at the top limit of travel or the one at the bottom. I ultimately used two but believe it should be at the top limit of travel. It would be great if someone provided a definitive answer.