[peterjon1]

I don't think it is possible to flood the interior from condensation on the A/C housing. If it were that, you could easily see the condensation by looking under the dash when the problem is occurring.

I think there has to be an issue with the drain through the firewall. I know you said you had cleaned it 3 times, but that's the only place to get a good volume of water coming in from the A/C, unless there is a crack developed somewhere.

Is the rubber tube on the firewall in place to direct the water downward? If not, air pressure could be forcing the drain water back in.

I have no idea if this has any bearing on your truck, but my '06 Stratus had a flooding problem which was caused by improper design of the drain. My problem was that the drain went through the firewall into a rubber tube that was attached to the firewall. Water would back up in the tube because the hard plastic tube from the housing did not form a seal in the rubber drain. Water then filled the rubber tube and entered the passenger compartment through the foam soundproofing around the drain, so in hot, humid weather I had the same flooding problem. Chrysler had a tsb for a fix, which ImperialCrown kindly sent me, but what I did was to remove the rubber drain entirely, and use rtv to seal the opening where the drain comes through the firewall.
Then, to direct the water down and out, I attached a 1/2" pvc street elbow to the drain, and then used 5/8" id plastic tubing down to the bottom of the firewall.

 

[peterjon1]

I believe that it's possible. If it weren't, home air conditioners and car air conditioners wouldn't have condensate pans and drains.

Yes, the condensate is what causes the flooding. What I was referring to was his comment that it could be condensate forming on the outside of the hvac box under the dash. That would not accumulate enough volume of water in itself, if indeed any condensation occurred there at all.

 

[peterjon1]

Curious - When you say the vent drain tube is clear, and the vent screen is clear, I think you are referring to the cowl air intake below the windshield, and it's associated drains on each side of the cowl, and maybe one in the middle, not sure about that. The A/C drain is a different animal, on the firewall directly in front of the heater box under the dash.
If you run water through the vents by the wiper arms, and you see water coming out, those drains are NOT the A/C drain.
The symptoms you are describing are those of a partially plugged A/C drain.

 

[peterjon1]

You can probably get a good idea of what is happening by removing the carpet from the passenger side while you are troubleshooting. You probably can then see any dripping/leaking spots easier. The carpet will have to come out to dry the backing anyhow.
In my car, there is foam sound deadening sandwiched between the box and firewall, and going down into the footwell. If your truck has that, it absorbs a lot of water. Just cut it out with a utility knife and set it outside to dry. When it is time to reinstall, stick it to the floorpan with some rtv, then use duct tape on the cut seam to hold in place until the rtv dries.