It's part of the seat back. First, remove the seat bottom (two release levers, press in and pull up on the seat bottom. Remove the seat bottom to reveal screws left and right and possibly center, holding the bottom of the seat back. Remove all screws. Push the seat back up (it is hung on some hooks at the top) and pull forward and it will come out. With the seat back out of the car you can see from the back how the armrest is attached.

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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