The price on the Rear Window Regulators has more then doubled since I wrote this thread (thread not for everyone).
Extreme Window Regulator Repair
Extreme Rear Window Regulator Repair,123 - PeachParts Mercedes ShopForum
Over on Benzworld someone had a threads on using W126 Rear Regulators. For some reason on the Car they do not get bent up like the W123s do and you can find some good used ones.
Added; I found the thread:
Using a 126 window regulator on a 123.
http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1598364-w126-window-regulator-into-w123.html#post4812820