In my case it was a $4 plastic slider.
I illustrated the fix on my 1983 300SD on my web site.
Go to Cars - Technical - Chassis and look for Window Regulator.
If you can get your door panel off (there will be more to unplug on your model) you should be able to do the repair. I dread to think of the cost of having a third party do this, not to mention the high probability of filty thumb prints everywhere.
No special tools required on my model.