I've done both rears on my W124 and replaced whole assemblies for about $140. The fronts are more expensive.

Usually the track assembly components fail, not the motor.

You can remove the door panel and disassemble or replace the offending part(s) yourself.

I believe the parts are now available (for a few dollars instead of hundreds), so you may not have to go out and buy a whole assembly.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car