Pictures below show the cause of the water leaks my truck had. Of course, the water ends up pooling under the carpet or vinyl mat and soaking the carpet pad.
If you can see signs of water leaks on the inside of the firewall up under the dash, the problem is cracked and/or thinly-applied seam sealer underneath the cowl. In my truck, the signs of a leak were very easy to find on the drivers side above the fuse panel, There was also a smaller leak on the passenger side to the right of the heater box.
The fix is to rough up the seam sealer surfaces, and then apply a new coat of seam sealer. (if you can remove the old stuff, more power to you!) I really like the SEM product. On the inside, I used fiberglass mat covered with POR-15 to seal the rust-through spots on the driver side. The hole on the passenger side was smaller, so I simply used seam sealer.. Not a professional fix by any means, but I have seen the fiberglass mat/POR-15 fix last >15 years with no rust-through.
Passenger side
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Deep dive into driver side
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Inside damage -- some near rust through in places above what's shown.
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I also had really bad leaks past deteriorated kick panel vent seals. And no, the passages were not clogged with leafs or debris. I think it was just a poor design that allows water to run too close to the vent.
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BTW, water leaks, however minor, are a good reason to use 3/8"-1/2"closed cell foam instead of jute or felt under your carpet or vinyl mat. A vinyl mat works great for me because, hey, it's a base truck.
