We had that problem on our 1997 C230. Bottom line was that pieces of leaves (despite my best effort) had worked their way in to the water ducts. We had standing water inside one day so I knew I had a problem. After reading on this forum, I opened all four doors and ran a hose full on (without a nozzle) against the windshield. I saw where junk was trying to flow out and used a bent shisqbob to dig out the crud. There is also a duct in the engine compartment down low below the windshield on the right side. I bet that is where your major block is. Use the water pressure flowing down from the windshield to see where it is blocked. It should be gushing out the engine compartment hole and flowing pretty quickly out the holes that are in the door jam area. When the water flows freely, you're done. Good luck.

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Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230