there is a welded seam where the door support, cowl support, and firewall all come together. this area develops pin-hole rust perforation and leaks into the cabin. the easiest way to fix it is to pull the cowl panel off, stick a floor jack under the opposite side of the truck, jack it up as high as you can, and pour about 1/2 cup of brush-on bedliner down the cowl vent. it will fillthe pin holes, and the excess will run out the side behind the fender. if you have big holes in the cowl, you may need to do this procedure two or three times to get sealed.