Had the exact same situation with my 94 e320 Wagon with 130k. Do a search on "windshield wiper" for more info on CLEANING the wiper arm mechanism.

Briefly: The actuating arm collects crud and prevents the arm from moving properly.

To clean it:
1] Turn igintion and wiper on any speed and turn off ignition when arm starts swinging. You can then manually swing the arm to any position.
2] Remove cover rectangular shaped about 5" long and 2" wide with 1" sides. Apply pressure with palm of hand and slide it down. There are moulded slide on inside of each side. If you break the sliders, you can buy another cover from MB for $9 US.
3] Use a rag and trans fluid (your trans won't miss a few drops from the dip stick) and clean the actuating crome rod as you move the wiper arm accross the windshield.

Use the money you were going to spend on the relay (located in main fuse box) and buy yourself and significant other MB keychains.

Happy Holidays.
Bob