I too just went thru this - 126 model - drivers side rear door.

Following the advise of another member here, I used a clevis pin instead of battling with the rivet that must be peened.

Dremeled off the old rivet with arm extended downward. Drilled a 5/16" hole thru the spot where the rivet had originally existed. Bought a 5/16 clevis/2" long from Home Depot. Cut it down so there were 2 holes left on the clevis. If you don't do this, the pin will collide with the window. Ran it thru plastic bushing(sliding jaw), then thru hole in arm and secured with cotter pin. Good as new and no need to remove the regulator, motor, arm, etc. and battle with peening a rivet that will likely cause you to break the $10 plastic bushing.

My 2 cents.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"