New Regulators at one site were $550+ each.
The Rear Window Regulators on My 84 300D are made of the Pot Metal. That Metal bends and then the teeth on the Regulator Gear wear out at a bad angle.
I don't know if it is possible to fix yours the same as I did one of mine as yours is different.
Extreme Window Regulator Repair
Extreme Rear Window Regulator Repair,123 - PeachParts Mercedes ShopForum
I was able to straighten out and repair one but while I could have straightend out the Left Rear it would have done no good because the Teeth were too badly worn. I had to buy a new one at $150 (since then the price has shot up to $250 each).