Does the VehiCross service manual show any likely locations\parts that water would be likely to accumulate, like air-flow ductwork or heater core enclosures? If you can hear a sloshing noise coming from the passenger side dash after it rains, it shouldn't be hard to at least narrow it down to likely accumulation points. Most vehicles have some kind of air intake for the ventilation system up under the cowling that houses the windshield wiper motor and wiper connecting arms. Maybe water enters at that point due to a poorly designed intake position, or intake seal, and works it's way down from there.

I haven't been able to get the VehiCross manual that I downloaded to open, or I'd check out the ventilation system layout myself. When I try to start the program, it always asks me to insert my Isuzu CD-ROM.