I've had the same problem with my '83 300D and it was the windshield gasket. It was leaking along the passenger side windshield post. I confirmed this by removing the carpet and the kick panel and had someone move a hose around the windshield. When they got to where it was leaking it poured in. There was no question as to where the gasket was leaking. I used some silicone windshield sealer which flows easily and it has taken care of the problem. I treated my windshield 2 years ago.
Candidate Mid-Atlantic Regional Director
Virginia Section MBCA