There is a little tap or something so you can bleed the heater. The seal can fail sometimes. I'm sure it's on the passenger side.
Also did the water appear after heavy rain ????
If so it could be:
1. The sunroof drain holes blocked. The water then runs down the inside of the door trim and into the footwell.
2. The door seal
3. Drain hole under the black plastic that covers the wiper mechanism. The drain hole gets blocked with leaves etc. then the water runs down the bulkhead into the passenger footwell. The carpet will be wet quite far up into the dash and the ECU may be wet