Larry's right about checking the underhood drains first thing. Right after I got my car, I noticed a bunch of water in the right rear floor. Cleaning the drains solved that problem right away. But during heavy rain I was still getting wetness in the front driver's footwell. This problem, unfortunately, did require replacement of the front windshield seal. Heavy rains also brought some water into the trunk. I checked the seals on the taillight units: one was not only torn and brittle, but also not even seated properly; the other was entirely missing! Two new taillight seals helped a lot, but there is still some drippage from what I suspect to be a bad rear windshield seal.

Good luck,
Dan

'85 300D