If the coolant has good color and circulation, then the problem isnt a dirty heater core...but you might want to check and see if the heater core has been bypassed (some people do that to avoid having it fixed then they forget about it and sell it...)
Just open the hood, walk to the passenger side of the car and look down behind the engine...there should be two black radiator hoses going through the firewall...(from engine to core and from core to engine)
If both hoses seem to follow the path I mentioned, then the core wasnt bypassed, which means the problem lies eather in the control panel or in the air select...but I dont know much about that stuff, so someone else would have to post it...
