sounds like you might need to get the A/C drain elbow that FORD realized these trucks should have had from the factory. the A/C drain is in fact in the passenger side of the firewall, and it is poorly designed. if you look at the firewall just above the passenger side exhaust manifold, you will see the drain tube (it is MUCH easier to see if you take the fender liner out). this tube is in a horizontal position, and only entends beyond the firewall by about 3/16-1/4". so, as you drive down the road, the moving air will cause the condensate to flow along the bottom of this shprt tube, rather than drop to the ground. then what happens is the condensate will soak into the padding where the tube penetrates the firewall, and then will continue to soak into the carpet padding on the passenger side.
long story short, you can put this elbow onto the drain, and it will force the condensate to drip onto the ground. its part number is F3LY-6A614-A, and it costs about 6.00 from FORD. It can be a little tricky to install, but seems to do the trick. i removed my carpet a couple times to dry it all out correctly before i finally found the culprit and fixed it.
here is a picture to show you where it is located. i had put a temporary elbow at the time until my dealer got the correct elbow.
i hope this helps you out!