After all the rain we'd received in South Florida over the last few weeks, I was lucky to discover & diagnose this issue quickly.

At first I thought my passenger-side floorboard was soaked from a clogged A/C evaporator drain (which I knew can happen) after a few-hour drive a couple days ago, but after doing my best to reach & clear out the drain "gutter" on the firewall behind the engine and vacuuming out the water on the floorboard, the very next morning the floorboard was soaked again after a night of rainshowers.

Figuring then that the rainwater may be coming through the A/C fresh-air intake under the cowl below the windshield, I checked to see if the lower cowl-area drains were clogged with debris (causing water to back into the air intake), but they were clear also. The drains along the lower lips of all doors were clear of debris too, all door weatherstripping looked good, the windshield sealant looked fine, and I even re-sealed the high-rear brake lamp and A-pillar screwholes holding the side-windshield trim strips in place...But water was still entering the cab from an unknown exterior source (which I tested by "showering" the truck with a hose), and I hadn't yet been able to tell from which direction it was getting to the passenger-side floorboard.

Then a breakthrough: After removing the passenger-side kick panel cover, I found that the inner kick panel cavity was full of water up to the first opening! As the rain filled that up and kept going, it would have begun overflowing right onto the passenger floor from not only behind the kick panel cover, but behind the insulating panel.

So how the hell was it getting in there? Just exterior to that cavity is the forward door jamb, and all the rubber plugs/door wiring conduit tube and hinge nuts were intact and confirmed not to be leaking with the hose. Bringing the hose higher and over the front-passenger side of the lower windshield cowl, that did it...water immediately started pouring into the kick panel cavity! The leak had to be somewhere in front of/below the windshield cowl running down over the passenger-side firewall.

Removing the fender liner for access, I noticed a large seam between two sections of the passenger-side firewall where, if not well-sealed, water could have easily gotten into and behind that corner of the firewall...And it looked like there may have actually been a hair-thickness gap that opened up along it under a bunch of wet crud. One easy stream from the hose aimed at that exact joint...Bingo...Water immediately streaming into the kick panel cavity, which would have overflowed onto the floorboard if allowed to continue.

I cleaned all the crud out, allowed the section to dry, then oozed a bunch of contact sealant over the whole area and let it dry. Tested it again...leak gone.

So anyway, to make a long story long, if you're fighting a soaked passenger-side floorboard after a rainshower and can't figure out why after eliminating the usual suspects, you may have a failed firewall seam.

Here are some photos to show exactly how the water is flowing (the red arrows) and where the bad seam is (the blue line):


Flowing between the windshield cowl and the hood:










Flowing down between the firewall and the foam piece that helps to insulate the door jamb:



With the fender liner and the foam piece removed, here is the failed firewall seam, highlighted in blue:











And finally, inside the cab (seen with the side-wall insulating panel previously cut out to make room for satellite-radio tuner equipment), the water starts from inside the top of the kick panel cavity (behind the far-passenger side of the firewall), down into the basin of the cavity, eventually filling and rising to the level of the first cavity opening, then overflowing onto the passenger-side floor from under the carpet and insulation:



Hope this helps anyone with a similar problem.

 

I had the same problem. The fix worked great. Thanks for the post.

 

nice write up!

 

I had the same problem in my 2005 Dakota on the driver side and after a complete disassembly and sequentially running the hose in higher up parts could not clearly identify which seam was leaking, there were a lot of them! I went crazy and sealed every seam I could see with some automotive urethane seam sealer and with the running hose test there is no more leak.

 

My passenger side leak came down through the HVAC system, leaking directly out of the bottom of the fan enclosure. I tracked this down to water leaking into the system around the fresh air intake - it sticks up like a smoke stack through the body panel that forms the floor of the large area below the cowl, below and in front of the windshield. My truck was parked on a slope, nose up, so there was pool of water around the intake stack and it leaked in. I sealed it all up. This picture shows the intake, it is very hard to see even with the cowl cover off, as it is tucked up under the base of the windshield. I first used expanding home insulation spray foam called "Great Stuff", but later learned it is not waterproof so I surrounded it with some urethane seam sealer.

It was cool to disassemble the fan and look up into the HVAC system, it is a direct shot straight up to the air intake. Also the fan is designed to handle and divert and drain any water that comes its way and drain it out of a hole on the very bottom of it.

 

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