I'm having a similar problem with my 300SD, all don't work except the rear right. However, the soldered leads on the motor tested OK for power so my motor is bad. Did you test the leads at the motor? I'm assuming they are not getting power. I'd trace the wires back to see if they are mangled anywhere. After 30 years of loyal service, those things can take a beating (and from what I hear it is a fairly common problem, at least in the w126).

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth