I got some light and a mirror into the door so I could watch what's going on. I can now see that the X-mechanism that moves the window is connected to a long metal arm. That arm comes down next to a big gear. This gear doesn't make a complete circle, it is only a segment of a circle. It looks to me that the arm is supposed to move up and down, carrying the window with it, as the gear is rotated by the electric motor. If this is so, then the "POP" that I heard was the breaking of whatever part connects the gear to the arm. Is that part replaceable or do I get to buy a whole regulator?

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970