Not rear defrost, your regular defrost, again, when you use defrost (front defrost as in for the windshield) your a/c compressor kicks on to take the moisture out of the air. That moisture then has to leave the car through the drain tube for the a/c. I have no idea where this would be located in your particular vehicle.

EDIT: The reason I don't think it's your heater core is because you say it's straight water, if your heater core is leaking you will leak coolant into the inside of your car. Check you coolant and make sure it' snot straight water, if it is you're going to fuck your shit up when it drops below freezing in your area...