This spot is a favourite entry point for rain or car-wash water and depending on parking position, road camber and whether RHD or LHD will manifest itself in either the driver’s or passenger footwell.
It drove me crazy finding out how water was getting into my footwell as I too checked every possible entry point and came up ‘dry’ …still, there it was, wet carpets.
On my LHD car, parked at the kerb on a flat section of the street, the driver’s side was slightly higher due to road camber. When it rained, rainwater ran down the windshield and into the gutter at the bottom of the screen - normally, right?
Now this gutter, which continues for a short distance (3"- 4") under the fender then ends abruptly, is designed to let the water drip down harmlessly into the wheel well and on to the road. That was the theory, and indeed when the car is moving or on a level surface, it works as designed.
Unfortunately, when the car is not level, gravity lets the rainwater fall on to the electrical harness directly below and wends it’s way along the harness to the big 2-1/2" grommet that brings the wiring to the fuseboxes. It then seeps around the seal and gets drawn inside the car.
In a wet climate, this grommet area being almost continually damp will eventually rust into a large hole - as I’m sure many owners in the UK have found out.
On my car, I found the problem before it rusted and soon after I stopped parking under cover. The way I ‘fixed’ the problem was to take a 6" length of 3/4" rubber hose, cut a 2" slit in one end and “impaled” it on to the gutter end, zip-tied to the sunroof drain tube to keep it in place. I also cleaned the grime from the grommet mating surfaces and added some gipe to further ensure it was sealed up.
With this kluge in place, the drips don’t eject from the gutter any more but move along the hose to a point a few inches beyond the harness where they can find their way to the ground unimpeded.