You will need to lift the car enough to get under it support it properly. look between the engine and the firewall, you will see a rubber tube coming out of the firewall. it will be somewhere between the center and passenger side of the car, you can run a clothes hanger up the tube and see if you can unblock the plug. You maybe able to put the end of your shop vac over the opening and suck out the debris. Run the A/C or defroster and watch for water to flow from drain if so you fixed it. If not you may need to have someone remove the heater box to repair further. Good Luck!