1995 Dodge Truck A leak

Hello,

I was handed down my mother's old dodge ram and for the longest time it has had a mildew smell to it and when it rains or we wash it we find that the passenger side carpet is wet. One day I pulled back the carpet and noticed that it was wet all the way up and as far as I could tell was leaking from the firewall. I checked the windshield and the passenger door seals and thought they looked find. I heard that it could be the AC drain hose? Or maybe a Water pump. Thanks for helping out a rookie.