Hey, there's two places to check for water leaks, the heater hoses that come into the interior through the firewall, and the windshield. There are a few hoses down by the heater that
could be leaking into your interior, I had the same problem once. Also, the heater unit itself could be leaking somewhere, turn your car and your heater on and see what you can find.
The windshield around the corners also leaks sometimes, might want to check that out. See ya