Typically caused by a grommet missing in the bulk head, usually on the near-side. When the car is parked slightly up hill the water will happily drain into the passenger footwell. Easily solved with a suitably sized grommet or silicon.

Alternitivly check the lower corners of the windscreen for leakage round the seal. This can be fixed with use of silicon seal otherwise the screen will need to be removed, cleaned or a new seal put in.