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If you have water coming out of a vent from the heater box then it is almost surely a plugged heater box drain tube. The heater box has a drain tube which comes through the right side of the firewall to let out condensation that forms and drips off the evaporator. If the tube is plugged with debris then it will cause the heater box to fill with water until it reaches the lowest point where it can leak, which in your case is the vent.
where is the condensation line drain tube located ?