My 2000 GMC Sierra is getting a musty, foul odor inside the cab. The same thing happened to my 1989 Chevy pickup a couple of years ago. When I turned a corner, I could hear water splashing around somewhere under the dash. I think that the problem is rain water that runs into the grill in front of the windshield, and gets trapped somewhere in the ventilation system. There must be a water drain somewhere, and maybe it's getting blocked with leaves . Where do I look for a drain to unclog ?