I recommend first, try to flush the heater core itself. If that doesn't help, then my next recommendation is to have the blend door checked. THe blend door determines if you get AC or heat to the inside. If it is sticking or not working correctly, it may be stopping the heat from entering. It is located near the heater core itself.
Let me know what you find.
Joe
Wednesday, November 25th, 2009 AT 9:30 PM