Well, I'd first check the hoses on the firewall that go into the heater core. Perhaps one is leaking and it's flowing into the ducting?
What you can do to test and see if it's teh heater core is to take a piece of metal pipe and slip it into the end of the heater lines so they connect together. This will cut off the heater core and shoud stop the leaking. Also, it's a poor many way of driving the vehicle without it leaking while they are too cheap to buy a new heater core.
I can get you some stuff on getting repairing the problem though. Just give me a day.