I'm prepping my car to be sold in a month or so, so I'm trying to fix the coupe specific problems that the cognoscenti may ask about. On to the DIY:

Keywords: w123 coupe, 300CD, 280CE, coupe window, rear window, halfway, coupe window regulator, 1983, C123
Summary: cleaning and lubing the tracks for rear (quarter) power windows in the W123 coupe. Car in pics is a 1983 300CD
I recommend using the Factory Service Manual for this repair since there are several overlapping areas.
You will:
Remove the rear seat (FSM 91-170)
Possibly remove the rear hat shelf/parcel shelf (68-458)
Possibly disconnect the front seat belt(s) (91-500)
Remove the upper rear quarter trim panel (68-449)
Remove the rear quarter trim panel (68-456 – caution: not correct and may damage parts)
Remove the inner window wiper (68-456 – see above)
Remove the window rail (67-400)
Remove the rear window (67-400)
Possibly remove the regulator (67-415 – caution: outdated info)
Adjust the rear window (67-400)
And then put it all back together.
A helper, or a third arm if you’re an alien or mutant would be useful.
Before you start, keep in mind that the coupe is different. The vacuum controlled seats have a relay behind the glovebox that is energized to the battery and turned ON when the doors are open. If you leave a door open for a long time during this repair, that relay gets very hot. If you don’t open the doors, you get very hot. I recommend you lower the windows and get a fan and keep the doors closed. You need the battery and switches in place to move the windows. This repair is also MUCH easier if you can get your windows all the way down to start. A little lateral and downward pressure can help the motor overcome the friction in the rail.
Removing the seats and the hat shelf are self-explanatory. Why the hat shelf? Because the corner of the upper and lower side trim panels tuck under that raised corner of the shelf. And when you finessing the pieces back in you’ll wonder “Why didn’t I take out those two screws and get this thing out of my way?”

Why the front seat belt? Look at the pics. The plastic outside piece, the chrome strip and the vinyl interior are really one big piece. Sure you can pop them apart and hope you’ll get them to stick back together. On one side of my car they were already popped loose, so I didn’t need to pull the seatbelt out. But there’s no way I can get them to stick back together.