“I was washing the windshield on my 87 300E and I raised the single-arm wiper to the up position and now I can't get it back down.”
Same exact thing happened to me today on my C230. Tons of dried salt spray/road grime had it jammed pretty good. I got back to the office (quite embarrassing with the wiper sticking out) and grabbed the spray Pam from our kitchen. I think it was the latch that catches the arm from rising up too high. The Pam did the trick nicely and did not leave too much to clean up.
1988 300 SL (5 Speed)
1994 E320 Wagon