Sounds like one of two things may of happen. The good news is I don't think the heater core needs to be replaced. First you said that after you replaced the thermostat that you have no heat. Make sure the coolant system is completely full. Turn the heat on with the the car running untill full operating temperture is reached. Check that the coolant level is good. It may of dropped a little when the tapped air in the system came out. This will cause the coolant not to flow very well through the heater. If this doesn't correct the problem then remove both heater hoses to flush the heater core. It may have become clogged and needs to be flushed which can be done with a garden hose. Take notice to see if the water is flowing good and continue to flush untill no more debris comes out.
Hope this helps Backyardmechanic :)
Sunday, December 3rd, 2006 AT 7:45 PM