A tip for 309 owners:
My 309 hatchback collected water behind the passengers seat, on the floor. After an extensive search I found out, that the tail lights were to blame for it. The water collected from the roof came down as per normal via the gutter next to the rear window, and normally it should run away over the tail light. It didn't. Much of the water came into the car, because the rubber seals from the tail light leaked, and the water found it's way into the interiour. I had to take the tail lights off, clean and dry the frame, and rebuild the tail lights, using transparant silicone sealer. Problem solved.