1983 380SL, 2000 S430, 1991 420SEL (retired) - RHD
I recall the window rails are curved to allow for the curve in the glass. The block should guide it up the rail in the correct position. The glass is adjustable in all directions and may be out of adjustment thus hindering a smooth movement.The rails are partly adjustable. Adjusting the glass can be a real PITA........up, check,down, adjust, up again ....I was thinking that without the aft slider bushing that the window is crooked when being pushed up, so that the glass isn’t square inside the two channels.. I’ve only just discovered that the slider was a problem and I want to see if fixing that puts less strain on the motor before I replace what is probably a good motor.
If the slider is missing you may also get scratches on the glass as well as a rattle.