what exactly is the window doing??? sometimes, with these regulators the slides simply go bad. I lucked out and on both my rear windows on my 300SD, it was only my slides that were bad. I got quoted 200$ to fix from a shop and in about an hour and 12 bucks later i'd fixed it myself. I"m pretty sure your car has the lifting arm style and not the cable drivin ones. I'm sure someone on here can give you an idea of how to get that door apart, i can have mine on my 300SD off and back on again in 5 minutes flat, but the W123 may be differant. Hope this helps,
Ryan

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83 300SD.......sold
96 integra SE....sold
99 a4 quattro....sold
2001 IS300.......sold
2002 330i.........current.
2004 highlander limited....current.