Yep, Hitman and Allen got it. Its the AC condensation leaking into the cab. I had this happen on my '98 and it was where the assy snapped together. It should be draining ok and that is the first thing to check like they said. If it can't drain easy like the tube is blocked or clogged a bit, it will back up and over flow the drip pan and leak down the firewall soaking the padding.... Let us know how it goes.
Mike
'02 1500HD, 6.0L, magnacharger, 2000stall, custom magnaflow with electric cutout, E-fans, Nelson custom tune, Auburn LSD.
'79 L82 vette, Summit 355, Dart Pro1 heads, Comp 274 cam, Holley TBI, Built TH350 with 2500stall
'93 Blazer, full size 2dr 4x4, new project.....