I'll put some seasoning on my foot because it always ends up in my mouth when I contradict Steve -

You can take a part an extending wiper assembly. Basically there's a circlip at the driven end of the shaft that keeps it together. Inside you'll find that the shaft drives a gear that rides on a cogged rail on the inside periphery of the case. I suspect the wiper is out of time because the cogged rail is missing some cogs. Alternatively the gear can be missing some teeth but I believe all 124s use a metal gear vs nylon or fiber cogs. [whine] This is unlike the 140 that uses a nylon gear with a weak splined fit on the shaft. [/whine]

I've looked at a few extending wiper mechanisms in my search for a replacement gear (no other MB has the same gear as a 140) and I've found that all cogged strips can be replaced. Finding a replacement is the problem. You can get a used wiper assembly, but if it works, why take it apart to fix yours? Which I guess is Steve's point

BTW, If you remove the gear or coged rail, you can limp along with a non-extending wiper arm.

Sixto
95 S420
87 300SDL