After wrestling with squeaky windshield wipers for years, I found a TSB on this. The problem according to MB is the angle of the wiper arms. They should be 2.5 to 6.5 degrees from the wind shield when parked and -0.5 to -3.5 degrees when in the reversed position. Same angles for left and right wiper. Per the TSB, (thanks Konigstiger) there is a special tool to measure this and special tools to twist the wiper arms.

It looks like two big adjustable wrenches should do the trick for twisting (tweaking) the wiper arms. Measuring the angle seems trickier but basically, the tool balances the cross bar that supports the wiper blade on a frame with an angle scale resting on the glass.

I'll try and report back on how to do this and whether I found it effective.

Regards,
Peter