Rear heater on 1994 GMC Safari

I just replaced the water pump in my van. Checked the hoses, thermostat etc. They are fine. I keep getting a hissing from the rear heater, and the carpet gets wet and steam escapes from the rear heater. I'm putting 2 gallons of water/coolant in it every day. What could be the problem? Is it an expensive fix? Thanks.

Asked by for the 1994 GMC Safari
sounds like your rear heater core has a leak