Water leak into floorboard

[Rickie Williams]

I own two C4500's ...a 2003 Chevy 2 door and a 2006 GMC 4 door.  Both trucks leak water into the floorboard. Seems to be near the kick panels on each side, but sometimes seems to be a function of whether the trucks are parked on level ground or not.  On the 2003, I have completely disassembled the vent panel at the bottom of the windshield and searched for any stopped up drain and found none, then I disassembled the heater core/HVAC box inside the cab as well...and once again found nothing wrong.

 

Any ideas other than to never let either truck get wet?