There are some threads on windshield leaks, which are probably more comprehensive than I can describe here, but I think it comes down to a few things:
- Make sure the cowl seals are tight to prevent water from coming in via the cabin filter
- water may leak onto the passenger's side but you won't notice it because the carpet padding will soak it up until it's really bad
- Look for windshield leaks
- after a heavy rain or car wash pull the plastic covers in the front footwells between the seat/floormat and the door and look for water
- Check for water or signs of corrosion (green discoloration) in the terminals of various wiring harnesses
- Clear your sunroof drains
- I think this may look similar to a windshield leak so just because you see water above doesn't mean its a bad windshield seal