I have a 1991 300d and have issues with the pass window rolling down. The switch is good. The motor has power at both posts and when I removed the regulator/motor it worked both ways. The window only rolls up! When I switched the posts it only rolled down. I dug deep into the forums and can't find an answer. I also tried tracing the wires from the switch to the motor and found A LARGE CONNECTOR with nothing connected in the place where the factory alarm control should be underneath the pass side dash on the floorboard/ firewall.
Could it be that the previous owner removed the alarm control or did some models not come with alarm control box? If they did remove it, could that be the reason my window won't work?
I looked at a 1990 300e gasser set up at the wrecking yard the other day and realized where the mystery open plug on the left side pass foot well connected to, but I didn't have the pug on the other side close to the door that is mounted to the floor and red in color. I'm confused and am desperate!

Please don't tell me it's the switch or motor. I tested the switch on the drivers side and it worked. I put the drivers switch on the pass side and the same problem continued. I pulled the regulator and ran it on the drivers side posts and it works both ways. It's not the switch or regulator unless the switch on the pass side is different than the drivers. Would the pass switch work on the driver side, but the driver side won't work on the pass side??? Maybe because of the automatic roll down feature on the driver side??? Maybe the previous owner put two drivers side switches on both front windows when it is only suppose to be on the driver side??? I have nothing to go off of!

This might be a problem with the convenience module. It is a module (I believe) under the back seat that is responsible for allowing the windows/sunroof to be opened/closed by key. I don't really know much beyond that though. Its the only thing I can think of that is between the switches and the window.

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(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


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You can trace the wires and see where they go - Compare with same cars in wrecking yard (almost impossible except to reconnect disconnected things.)

Or best is to find a wiring diagrams manual or pdf for your specific year / model car. The pdf for my 126 Mercedes is 164 pages long of just electrical diagrams for everything.

I don't know about for your car, but try searching google.com for...
mercedes 124 electrical diagrams

If you find a link with startekinfo.com, that is a Mercedes link and would be best - might take a bit of hunting...

If all else fails, try a Mercedes dealer ($$$$$$ Get out you wallet!) or alldatadiy.com ($$)