Ford Focus condensation caused by water leak

Problem with condensation on windscreen has been attributed to a water leak. The water is in the footwell on the drivers side. I took the car to the local dealership who did a full range of water leak tests on the car but were unable to find any leak. They dried the car out and returned it but within 48 hours of getting the car back the problem had returned and the carpets were again sodden. The car was parked out in the rain during this time. While I had the carpets up I did notice early corrosion to the electrics. I am still paying for this car but I have been told that water leaks are only covered by the warranty for the first year. If the dealership is unable to find a cause where can I go from here?