pull the carpet up and check if it dry underneath. the carpet has inches of foam under it and it can hold water, or coolant just fine, while hiding it. you need to pop the door sill molding up, and the front plastic kick panel to show the hooks the carpet is held on to at the edges.

you can also check the outside heater hoses, one of mine was all swollen and leaking down the outside of the firewall, and causing fogging of the windshield as well.