Originally Posted by tomw

You could lay a dry towel or shop rag on the 'shelf' behind the seat. If you do it with several pieces that are not touching, you could eliminate those areas that stay dry, or the back window if they all stay dry.
The under cowl drain could be plugged as suggested. It is pretty small, and I can testify that mine has a 'flap' held in place by 'christmas tree' plastic nails, that moves out of the way to let the H2O drain out. If you remove the top fender bolts under the hood, and the top bolt accessible from the rear by opening the door, you can swing the top of the fender away from the truck enough to see the drain. You can remove the flap and then run a bunch of H2O through the plenum intake to flush out any stuff.
Take a look at the underside of the cowl, where the antenna cable comes through. I had a leak there for years that kept the passenger carpet soggy. There are two pieces of sheetmetal sandwiched together by tack welds that stand up vertically. If the sealant is left off, water can dribble in there.
tom