I work for a replacement windscreen company and we compile data on the common causes of water ingress on a wide range of models. Invariably, the windscreen installation is blamed as the cause of wet carpets but more often than not the cause is something else. The list below is copied from our database for Discovery 1 and unfortunately was compiled primarily from trying to make mine watertight. Don't underestimate the amount of water that will come in through the bottom of the plastic inner membrane on the doors if it's not attached properly.
PROBLEM:
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Water leaks from windscreen area or into footwells. Several potential causes, not necessarily the windscreen.
SOLUTION:
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Common fault with welds in top corners of windscreen aperture can cause leaking. Corners need priming and bond smeared over them prior to installation of the windscreen to prevent water ingress.
If vehicle is leaking into either footwell. Remove scuttle and check welds to outer edge of bonnet hinges as these rot and allow water into seam which leaks out into footwells. Refer back to customer to have work carried out.
There is also a vertical seam beneath the top rear corner of the front wing which can separate and allow water down into the footwell area. Corrosion of the front inner wing by the A post is common; this allows water to run down inside the vehicle and onto the floor.
The drain holes at either side of the well under the scuttle direct water onto the inner wing inside the wheel arch. However, there is a horizontal bar running the full length of the inner wing. The water sits on this and the inner wind corrodes allowing water to run down the inside of the A post panel near the relays inside the car.
The front door seals also leak, particularly by the door mirror/window frame/door shell join. The water runs down the door seal and out onto the floor
The waterproof membrane on the inner door skin behind the trim panel will shrink back over time, leaving a gap at the bottom allowing water to run out onto the floor at the bottom of the door.
If the vehicle is fitted with air conditioning, there are two short drain tubes either side of the gearbox in the top of the transmission tunnel. If they have never been cleared, they are probably blocked with mud. This causes water to leak into the footwells from the centre console/heater vent area. The pipes are only accessible from under the car.
