if its wet now the only 3 things it could be is the a/c drip tray drain, the heater core, or the cowl.

keep in mind the foam under the rug is thick and will hold water for a long time and stink like hell if you dont dry it up.

if you cant tell where its coming from you need to take the rug out, and watch for the leak to happen, hose test, run the engine see if the core leaks, turn the ac on, se if its leaking then.