I had the same problem a couple of years back. Open the passenger door and look where the rubber gromet is between the frame and the door. The wire harness goes thru it. My gromet came out and it was letting water in. GET SOME GOOD SILICONE and put a bead around the gromet and stick it back in. let it set for a day and it should seal it up. This was my case.
Jamest
12 Sec. Club! SOLD
La Viuda negra/ a.k.a. Black Betty
LQ4 6.0L, WCCH HEADS, LEVEL 5 FLT 4L80E TRANS. WITH TRANS BREAK, YANK 3200 STALL, TRICK TURBO, TURBONETICS T-76 TURBO, CUSTOM INTERCOOLED, 68lb INJECTORS, CUSTOM S/D TUNE FROM NELSON PERFORMANCE, COMP CAM 212/214 .557/.559 116+2, SNOW METH. KIT, . ALL INSTALLED AND DYNOED AT NELSON PERFORMANCE: 5**?HP @ 5073rpms and 5**TQ @ 4389rpms AT THE WHEELS WITH 10 PSI OF BOOST. ALL 3/4 TON TRUCK 5435LBS, NOTHING TAKEN OFF, STREET TIRES, AND USUALLY 2 BABY SEATS. WANNA RIDE?
SOLD***