It is quite likely that the heater core is clogged. It generally gets full of debris that doesn't settle in the radiator.
Drain the system, and have a qualified repair technician remove the heater core. It is located inside the car under the dash, and can be cumbersome to remove. Make sure they put plenty of towels down to absorb radiator fluid as this stuff reeks and is difficult to remove from carpet.
Flushing out the heater core manually, assures free flow of water, reinstall then check for leaks. Refill with the proper 60/40 mix of antifreeze to "distilled" water, and leave the radiator cap or expansion tank cover off, while you run the engine to purge the system of air. Once the engine reaches an optimal temperature of 180 degrees, close the cover, and check for heat.
If the car doesn't have heat at this time, then either you were sold a defective thermostat or the heater door isn't opening up properly. In which case the vaccuum lines may be broken.
Wednesday, January 28th, 2009 AT 8:11 PM