Oh OK... I think you are right Pete. I misread the post. The sliders are a common problem on the rear doors. There are two steel balls that ride on a cahnnel inside the door. They corrode, wear out and jam and in the process tear the channel up.
They are cheap, available at any MB dealer and its a do it yourself job.
There are two or three 10 mm bolts holding the channels inside the door.
Ask if you do not know how to remove a door pannel. They are different from most other manufacturers and can be severly damaged if you do not know the procedure.
1993 190E 2.3
2000 Toyota 4x4 Tundra