[R129 SL] 500SL windshield wiper gone bezerk! Film at 11

[Cliff Sadler]

Driving in the rain Saturday, and suddenly the wiper starts making a racket.

turn it off, and after stopping, come to realize that the in out piston
motion appears
to be out of synch, causing the blade to contact the arm posts, and bind.

took the wiper arm off, so as not to hurt anything, and watched the piston
arm. It seems to be somewhat erratic in where it decides to move in and out
in the cycle.

Returning to the park position, sometimes it's all the way in, and sometimes
it is not.

What can I check to see if the assembly somehow came loose, and has rotated,
or something is broken?

Specifically, can't figure out how to get the plastic cover off the main
pivot point.

TIA

1991 500SL