Passenger side or drivers side or both? if just passenger side it could be heater intake although as a 2004 you should have a snorkel which are less prone to leak than the old style. I had a leak on the passenger side and it took me a VERY long time to find it, it was a faulty door seal. I'd try pulling the carpets back as much as you can and get someone to spray the outside with a hose while you look for leaks. The problem is some leaks show up in a different place to the actual point of ingress. good luck