When it rains hard, the passenger side of the

pollen filter in bulk head needs changeing leaks when it get old
well knowen

Check under the carpets there is often a drain plug in the floor well which may be missing. The other thing that can cause this is if you are using the aircon and the drainpipe has disconnected from the evaporator drain pan. this is a pipe that runs from the drainpan through the firewall (cab and engine wall) to the engine compartment.