I have a 2008 Silverado 1500 WT extended cab truck. I am experiencing what I have seen is a popular/common problem with these trucks, wet floor boards after the rain. I originally tried replacing the brake light and gasket since the old one was broken. No change. I tried replacing the rear vents between the cab and the bed. Better but still getting soaked. I was originally getting so much water in the truck that the module under the driver’s seat was corroding up so bad it was causing the truck to not start or operate incorrectly if it would. I later found the new gasket on 3rd brake light was not sealing up and had gaps, so I used a little silicon and sealed it. I am still getting water somewhere. Both the rear carpet under the seat at the door area and the front floor boards are getting wet. What else am I missing? I know/heard there is a vent that drains from the front windshield could be a problem, but I don’t know where this is located. I know that my passenger door will collect water sometimes after a heavy rain and I hear it sloshing for few days. My be undrelated, but just wanted to note it. What else can I try to stop water from getting in?
You may want to look in the trough below the windshield where the windshield wiper linkage resides.
This will require removing the wiper arms and plastic trim piece below the windshield,
I’ve seen debris collect in the trough blocking the draining of water, and factory sealants cracking, and rust allowing the entry of water into the interior.
Tester
In addition to Tester’s remarks, there should be drains at the bottom of the doors. These can become clogged. Usually easy to clear the drains.
If you’re not familiar with the door drains, here’s an interesting video showing common location:
I was looking at the front windshield and noticed a gap between the window. And the plastic cover. Could this cause a water leak?
No.
Tester
No, but there are also drains under there that you’ll want to check. You can likely get to them just by popping the hood. Sometimes removing the plastic cowling helps, but usually not needed (in my experience - this truck could be different).
This might be where your water leak is coming from.
My guess, the water is coming in from the front, after hitting the windshield. It’s supposed to flow through various channels and onto the ground. You can get an idea how it works by directing a garden hose on the windshield. Do you see water pouring out underneath the truck? See those leaves in post 4? That may be a clue to the problem. The paths may be clogged. Suggest to start by vacuuming all debris from the cowl area. Open the hood for better access too. I do that job routinely, as part of I changing the engine oil. On some vehicles (my own Ford truck for example) once these paths get clogged to the point water fails to drain, they can be very difficult to clean.
Keeping the drains at the bottom of the door open is important too, and fairly easy to do once you figure out a way to see them. I use a mirror. Debris can jam between the bottom of the door and the sill, another place to keep clean.