W123 is straight forward. Remove the door panel - take off arm rest, remove black plastic cover behind inside door opening latch and remove that phillips head screw then pry the bottom of the door straight out. Also remove the door lock push/pull knob. When all that is free push up on the panel. Next you will come to the vapor barrier (plastic cover between the panel and door). Start peeling it away at one of the corners. Take your time here because you will reuse it and don't want to rip it. Once that is out of the way you should be able to look around inside the door and see the regulator and inspect the slide.
On my 300D the "teeth" are shot in the area where the window is closed. I will eventually get around to replacing them. They work but just make noise when they get all the way shut.

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Jim