That's kind of disappointing about the motor not coming apart. I suspect Bosch changed the design slightly between 1986 and the mid 90s, so that's kind of a bummer.

That curved sliding piece does go in that body colored channel. When I looked up info for my SDL it gave the part number in the EPC for it, but gave no mention where it was installed. Both of mine were down in the bottom of the door, so it took some trial and error to figure out just where they went. It serves 2 purposes - it helps to guide the window and it helps to keep it from rattling or jumping the track when partway rolled down and the door is slammed.

If you have slides that are missing or broken, the extra strain on the motor could be what's been causing you grief.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]