if it's not switches, relays or fuses, it's... the motors. This, apparently, is a Known Fault.

Replaced all four on my 300e since buying it 18 months ago. Back rear only went down halfway when I bought it, within a few months the others got noisy and packed up entirely. The driver's side did fool with me a bit by making its own mind up whether it would work or not, and I replaced the front passenger motor anyway because it was making too much noise, despite lubrication.

Bad news is that the rear motors come attached to the regulator, which makes them a bit more expensive. Fronts you can just swap out the motor. I paid about $150 each for the rears and $100 for the fronts.

While you're messing with the doors, it is probably worth your while to take a look at the door stops. They tend to get noisy and crotchety with age. If they haven't been lubed over the years, they will eventually seize, as mine did, leaving you with a door jammed open down the supermarket.

BTW, if this door jamming ever happens to anyone, give the door one hard shove to see if it will close. If it does, climb in the other side and drive home to fix it. My experience tells me that you only get one shot at this. When I opened the door again to see if it was back to normal, it jammed up solid, to the point where the door stop started to bend instead of freeing itself. Had to unbolt the unit.

I've already replaced two doors stops, and will be doing the other two before too long. They are sealed units and 17 years is a long time on one shot of factory lube.

I've given up thinking of this as maintenance, makes me feel better talking about it as a restoration project.

good luck

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1987 300e manual 250,000 km (sold)