It sounds like the drain on the water collector is clogged. The water collector is where rainwater goes after it runs down the windshield. The outlet is on the passenger side, near where the battery used to be on older MB's. All you need to do is open the hood to the straight-up position, and look down below the area of the hood hinge. You'll find the drain outlet here. There's a rubber flapper thingy that gets clogged up periodically, attached to the outlet of the water collector. Don't try to un-clog it, just pop it off with a long screwdriver and forget about it.