On my 1991 300SE, I can make any either of the rear windows slide up or down by putting my palms on either side of the glass and pushing up or down. Not that I need to do much pushing down … gravity does that gradually, as I drive.
Today, I analyzed a parts donor car, so as to learn more about likely causes of the problem, and how to get access to the relevant parts.
The first step was to open the door. No problem there.
I removed the door switches. The donor car was a 560 SE or SEL, so the door switches have more features than on my humble 300SE, which doesn’t offer heated or adjustable rear seats. Even so, I pried the switch surround off with a flat screwdriver. One side has three fairly flexible tabs, and the other side has one large tab. I broke one tab on each side. With the switch surround out of the way, the switches could be accessed and removed, by pressing their catches inward with a flat screwdriver, and pulling the wiring plug away from each switch. I bought the two window switches; I plan to see if they’re the same as in the center console. If so, they’re probably less-worn.
I removed the door handle. With the window switches out of the way, a large yellow Phillips screw becomes visible. I removed that, and two similar screws in recesses in the door handle. With the three screws removed, the door handle can be lifted free.
I removed the door lock knob, by turning it counter-clockwise and then lifting it away.
I pulled the door trim away from the metal of the door; I own a tool for that but I hadn’t brought it along. Girl arm muscles worked well enough, as did digging in my nails around the edges to pry the panel away.
I removed several (non-identical) 10mm bolts and one or two 10mm nutes, and the window winder mechanism came loose. I chased its wires but couldn’t find an easy disconnect point, so I cut the wires.
I could maneuver the mechanism downward and then free of the single metal guide at the bottom of the window glass.
I’m not sure that I needed to get the window out, or remove parts in order to get the window out, but I did anyway.
I used a flat screwdriver to lift the flat, black horizontal door weatherstripping rubber that runs alongside the bottom of the door-and-window opening, on the inside of the car.
I used a flat screwdriver to lift the chromed horizontal door weatherstripping rubber that runs alongside the bottom of the door-and-window opening, on the outside of the car.
I pulled the window rubber surrounding the glass out of the car.
The rear window glass has two pieces: a rectangular piece and a somewhat triangular piece, to the rear of that. Separating them is a vertical metal pillar, part of which is painted body color. With the door rubber removed, a small Phillips screw is visible at the top of the pillar. After that screw has been removed, the pillar can be maneuvered free and out.