Its been over a year since I replaced the third brake light and I have not had any water in the cab since. That seal must have been my problem.

 

Oh ya it probably was your problem but there many others who have this problem with water entering the fan box and then the cab.
I am just saying how i think water gets into the box.

 

Not disagreeing with you just posting up my issue and its fix! The third light seal was bad allowing water to leak into the overhead and run down into the box. I am sure there are other ways it could enter but this was my issue. My issue was worse when facing down hill.

 

Oh that is interesting. I didnt realize that it ran from third brake light to box. I guess there are many ways water can get in a vehicle. When you think about it its amazing that water can be completly kept out of a vehicle.
H2o always finds a way in.

 

I never would have thought about it but I just installed an ebay cheap brake light and was not liken the looks of the seal. then just days later my wife had water hitting her in the feet and I could hear it sloshing in the box, so I replaced the light just to check and it fixed my issue

 

Wow quite a story. Just goes to show you, you never know!

 

So let me get this straight. The 3rd brake light on the back of the cab was allowing water to drain all the way to the front of the truck and into the fresh air intake where the blower motor resides? I just drilled a hole in the bottom of my box. 3rd brake light? Thanks!

 

I am having the same problem underneath the glovecompartment there's a black blower box that runs into the firewall. Where the firewall and blower box meets gets wet everytime it rains and has my passenger side carpet wet. Any suggestions would really be appreciated. But its definitely coming from where the blower box and firewall meets under my glove compartment. Any fixes please advise..thx

 

Thanks to Hurricane Hermine Id like to add some info to this thread as it took me a few years to diagnose a simple issue. First my leak was the heater box filling with water from rain and splashing out when driven soaking the pass floorwell. It was said by fleastang to drill hole in heaterbox, when I thought to only check the evap drain. Simple, pull back the inner fender liner and 3/8 hole and your water will pour out. Ultimately the incoming water from fresh air vent is difficult to reseal so this is a good fix. THANKS FLEASTANG

I also had water under the rear seats and after pulling the seatback and testing thinking it was the window it was the vents that let the doors shut easily. The foam seal isnt working any longer and coulnt get them out so I pushed them toward bed and silicone caulked them. Not pretty but works!

Everyone likes pics so>>>

 

Is drilling a hole the best/right way to solve this?