I have had the same problem with my 300 TE -92, (and maybe I still have, not sure yet). I have gone through all the drains, in front of the windshield and in the sunroof, but they were all OK.
Also AC drainage OK. I think in my case, this is a leaking windshield through the side seal. There wasnt perfect contact with the glass and the rubber seal. The windshield has been changed once. So I applied a sealant through all the windshield sides. I am not sure yet if it corrected the problem, it hasnt rained lately.
This is a really annoying problem.