One way to find out.

- Open the car door all the way.

- Roll up its window all the way.

- Take hose. Make the hose pour out water at a high rate, but not so high that it sprays and spurts. Just a nice solid gentle stream.

- Have girlfriend direct hose carefully at the OUTSIDE of the window so that the water rolls gently down the outer face of the window and into the slot at its base. Have girlfriend move the hose forward and backward along the length of the window so that the entire slot gets covered.

- While girlfriend does that, get down on knees and look at underside edge of door.

- Properly functioning door will have water pouring out of drain holes that release the water safely outside the door seals.

- Door with faulty vapor barrier may have the same as the above, but will also have water pouring out of the joint between the interior door trim and the metal of the door itself. It is this joint that is *inside* the car and results in rain getting your car floor wet.

- Repeat test for all doors on the car.

If you see the vapor barriers leak, remove door interior trim panel, and repair with tons of RTV silicone around all edges and all cracks/lifts/etc. in the vapor barrier.

If it passes this test, then definitely move on to look for other seals, including and especially the windshield seal, as Andy said.

If this floor-wetting issue tends to happen with the car standing completely still (for instance, it rains overnight and your floor is soaked in the morning), high degree of probability you're looking at door vapor barriers being faulty.

If the floor-wetting issue tends to happen only when you've been driving the car around a lot in the rain, then it's likely to be other seals like the windshield or door seals.