I just finished replacing the plastic piece in my 85 380SE rear driver side door window yesterday, following the official MB manual instructions. Not sure if same applies to your car, but if you have removed the door panel, and you need to replace this plastic piece, here's the skinny (from memory, as I'm not near the book or the car) on removal of motor/regulator assembly:
1. Window must be in full, or nearly, down position.
2. Remove the 1 nut and two bolts holding the assembly to the door panel.
3. Loosen top bolt holding left channel to doorskin, remove bottom bolt holding channel to door (channel needs to be swung out of the way to clear assembly)
4. Move assembly to the left and up to let the plastic piece on the end of the arm come out of the channel.
5. Manually raise the glass all the way up and secure with a wedge (body fill applicator works good).
6. Either cut the 2 wires to the motor (not preferred), or pop the cap off the back of the female plug and remove the cylindrical pins. The wires are all numbered to go back in the numbered holes, so don't worry if they all pop out (but pay attention to the motor wires - they're not numbered (but reversing them just makes the 'up' on the switch send the window 'down'. easily fixed, but test first before you replace everything)(none of this is in the book BTW).
7. Manouver the assembly out of the door, exiting through the big opening in the bottom. It's a pain, but it will come out!
8. Follow previous posts for testing motor or replacing plastic guide piece.
9. As they say, replacement is the reverse of removal

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1986 560SL
2002 Toyota Camry
1993 Lexus