I used to have exactly the same probelm with a Renault Safrane. However, the problem only occurred if it was parked on the left hand side of a road and facing uphill slightly. This baffled me for months, so eventually, one night when it was chucking it down I went and sat in the car for a couple of hours with a torch so I could trace the leak. It turned out to be water running down between the frame of the door and the windscreen pillar (like it was designed to do), but when it reached the sill beneath the door, instead of draining away out of the gap between the bottom of the door and the sill it was welling up and actually washing over the rubber draught strip and into the footwell. Of course it only happened when the car was at a certain angle, ie on the left hand side of the road where the camber of the road made it that the top of the sill was not sloping ever so slightly outwards, and facing up a hill allowing the water to run back from the door post and gather on top of the sill before washing in. Over the course of a good rainstorm I might bail out as much as a gallon of water. If your car is also on a slight slope perhaps you could try turning it around.