On my 2000 XJ, there is major water issues when it rains, even when not driving. Through the vent under the passenger side dash, when it rains, I get A LOT of water coming in. The foam rubber around the vent is rotted but doesn't look like it was ever meant to keep water out in the first place. So rather than stop water from coming in when it rains, I'd like clues as to where the main issue lies. Does not seem like the wind shield is leaking. The cowl cover seems good and tight. But I do believe I have a heater core and evaporator core leaks as well. But when the jeep is just sitting, and with the heater core and evaporator core being enclosed between the firewall and dash, could water from rain still be causing me this much problem?

What is the most likely cause of this situation?

 

If it's coming from the heater box, my first guess would be the seal around the air intake inside the cowl. Take a hose and spray a bunch of water inside the vent at the bottom of the driver's side wiper. Does it drip inside after you do that?

 

If the water is coming in that high, check the drain tube, coming through the firewall down at the very bottom where it meets the inner fender. That, and check the Cowl Drains behind the Fenders if it's doing it while not stationary. It's "normal" for water to get in these places, but there are drains to let it get out. Since it's not, they're clogged...