If the seal at the firewall has deteriorated, condensate dripping from the drain will run back along the bottom of the drain and track into the cab and down the inside of the firewall under the carpet.

I got a cheap 90° rubber elbow from AAP and pushed it over the drain, so the condensate drips down and not track back into the cab... problem solved.
Looks something like this;

 

On my old truck I pulled the blower motor/fan and dug out a 1/2 inch of mud from the bottom valley of drain area. You have to make your arm like a pretzel. I also have a rubber boot and it works very well.

 

Ok been there with my ram and my Dakota seal 3rd brake light with ultra black and all around
rear window should fix your leak and put the elbow on the a.c. drain I've not had any water in either of my trucks after this fix thanks and have a good day