When the pot metal frame bends the teeth on the regulator start to wear badly. The Motor Gear is much harder then the regulator teeth.
I replaced one with a new regulator but at that time they were $150 but several years ago they were $360.
It might be that by this time URO or some other aftermarket company makes them.
Extreme Window Regulator Repair
Extreme Rear Window Regulator Repair,123 - PeachParts Mercedes-Benz Forum
The thing with the contact points that breaks the circuit where the yellow arrow points.
Dorman makes front window regulators for W123's they may also have started making the rear ones.
In the next pic you can see the bend and with a closer look can see the tooth wear.