The usual cause of this problem is that the heater core is leaking coolant on the passenger floor. The solution is to replace the heater core.
There is also a condensation drain hose from the A/C evaporator; which sticks out from the firewall into the engine compartment behind the motor (and is concealed behind a heat shield in some models). This drain hose can become plugged, or can fall off. It is a nightmare to reach or service.