Diagnosis: Plastic sheeting covering the inside of the door, behind the door panel is leaking.
Fix: I replaced the plastic sheet with medium gauge vinyl. Vinyl has to be primed so that the urethane sealant will adhere. Maybe you should do a little test patch before you try to glue this into the door of your car. I used a self etching primer. I think this worked out well. The urethane is thick black window glass stuff in a caulking tube. It's kind of the urethane equivalant of RTV silicon sealant. I've applied this fix to my '89 Golf which has really horrible leaking problems. I took photos this past weekend! (Aug 15, 1998)
Sadly, a tube of Federated brand RTV silicon failed to cure. Very, very irritating. It set up to about the consistency of sticky, greasy cream cheese. It took a while to scrape all that off. I returned it to the store and got my money back, but they left the rest of the tubes on the shelf. Are these guys bozos or what? Moral: test your RTV first. It's a good idea anyway given the adherence problems with vinyl and plastics.
Tools: scraper, sharp knife, screw drivers, caulking gun
Parts: vinyl (from sewing supply), self etching primer, good Urethane or RTV sealant, duct or masking tape
Vinyl, primed, held in place with a duct tape. It was taped in place, and cut to fit perfectly before priming.
Caulked and pressed down. Do not start pressing in one place and work around - you'll build up a wrinkle. Press in individual spots alternating from one side to the other until all parts are evenly pressed down. Note the width of the seal visible at the rear edge of the vinyl is 1/2 to 3/4 inch (1 cm to 1.5 cm) wide.