[thstone]

Easy to check - have a helper sit in the drivers seat while you take a water hose and spray slowly around the door. When the helper sees/feels water coming in, you've found the suspect location and can investigate further to find the exact cause.

I had a similar problem in my BMW 325i with the rear seat floor wet and it turned out that the rubber door seal was no longer sealing even though it wasn't torn (just old and inflexible).