The radiator fluid needs to be topped off at the right level. The upper and lower hoses need to be REALLY hot when the car is warmed up, if not, it is most likely a bad thermostat. If the thermostat is good, then check the heater control valve to make sure that it is working correctly. how clean is your coolant? if it is really dirty and rusty, the heater core may be plugged up with rust and needs to be flushed out. Best of Luck!