The problem is not the window regulator. All that is fine.

My mechanic took out the comfort module, the thing underneath the rear seat, and if he plays with that, he can get the window to go up or down. I think he said somehow just that one window is not getting ground.

He couldn't fix it so I said I'd get a replacement part which I just got today. I put it in, and same problem. Left rear window won't go down.

Any suggestions?

1991 300d, 252k

Sounds like the harness ground to the convenience module is bad. Try adding a new ground wire to the harness plug ground terminal, see if that gets you going.

A bad ground to the module would cause ALL the windows not to work.

Your module should have a total of 3-4 ground connections. One fat one feeding the module main plug and smaller grounds to the outermost center plugs as well as another in the second large plug. If you look at the schematics you can see which one is in series with your inop window and do a continuity test between it and the window motor and ground and wiggle the harness to see if you get a break. You should also remove the door card and check the wiring thru the pillar to the motor etc, as well as the thermostatic switch at the motor.

If your mechanic got the window to work while playing with it, the motor is good, as are associated relays and if done with your old module, then technically the module itself is also good. Particularly if you replaced the module with a new one and no change was observed. The only thing left is the wiring and likely a failing connection in there.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

I'd pull the door panel and jumper battery voltage directly to the motor and try to run it up and down several times. While doing this move the wires in the black accordion boot conduit in the door jamb.

Leave that door panel off and carry a test lamp to see if it gets power to it when not working if you get it to work.

Broken wires in the door jamb are common as the car ages and the insulation gets brittle.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I had a similar problem; ended up replacing the window switches in the center console. problem fixed.

I took only the affected window switch out, replaced it with a known working one, but the window still won't work.