I have a 2001 ram 2500 diesel and I'm getting a substantial amount of water coming into the cab through the heater. This happens whenever I wash the truck or it rains. It is dripping out just below the heater motor. I have already replaced the motor and resistor once due to water damage and cannot find it's point of entry. Very frustrating. Also removed the cowling at the bottom of the windshield to locate this and I just cannot see how it is entering the cab through the hvac system. Please, if anyone out there has had similar problems, help me out. Much appreciated!

 

this may not be your problem, but a common leak is around the A/C drain on the firewall. its behind and under the A/C dryer, beside the pcm. its a tube about 3/8" diameter and only sticks out about an inch. the seal around this tube is prone to crack. you can reseal around it with rtv, and/or attach a drain hose to point it down a few inches. another common leak is for that tube to get plugged up. you can poke it out with a coat hanger or something.

 

Mine was leaking at the 3rd brake light and making the driver side floor wet, believe it or not. I replaced the gasket ($9?) now it's always dry.

 

Mine was clogged....used a clothes hanger to un-clog it.