I had this same thing happen to my 98 GT this summer. The passenger floor board was soaked. It turned out to be the AC condensation drip tube was clogged. The condensation backs up over time and runs into the car. This may explain why you are not seeing the water drip out side the car. This was a simple fix for me. Im not sure where this part is on the newer GPs, I assume in the same general area. The condensation out put on my 98 was on the firewall in the area right behind the glove box. It should be a little rubber black elbow shaped tube. You should be able to see it by looking under the hood...may need a flash light. It pops right off. Take it off and clean it out good. If thats the problem it should clear ir right up. Hope this helps.
