This isn't really BMW specific, but my guess would be that you have condensation build up in the HVAC module (where the heater core, evaporator and other stuff is). Under the hood, on the firewall there should be a tube that comes out and doesn't go anywhere. This is just to drain the HVAC module. Give it a squeeze and see if anything comes out. They can get plugged up with debris and cause water to build up in the module and leak on the passenger side floorboard.