There are a number of possibilities. One is the rear sunroof drain. It's much harder to get a snake down this drain (the front ones are easy). One experiment might be to use tape to seal the sunroof (make sure it won't stick so well as to pull off your paint) and then drive in the rain or use a garden hose. If it still leaks, it's not the sunroof.

Another thing to do is to lay under the car and look for drains. I don't know if all drains on your car terminate in a rubber boot or not so don't assume that you have found all of the drains by finding all the rubber boots. You can probe up into these drains (plastic line for string trimmers works well) or blow air or even water to see where the drain comes from. make sure the rear seat is out because some of the drains could puddle there. Have a helper to watch and listen while you exercise the drains from below.

I don't know if the air vent behind the rear door has a drain but it makes sense that it should, water could easily get in there. If the trim will come off you may be able to probe down and find a drain.

The easy fix for a drain problem is to remove the leaves and dirt that are clogging the drain. The hard fix is to clean and seal the rusted area that has occurred because the leaves and dirt weren't removed years ago. Keeping drains clean is one of the most important and most neglected parts of Mercedes service. A "drain list," model by model, would be a good DIY project for someone and it could be added to the wiki along with the vacuum diagrams.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970