There are a couple of things that have to be checked, 1, is the coolant level low, 2. Fell the hoses that go into the firewall to see if they both are about the same temperature rubber hoses for the heater 3. Is the temperature guage read mormal about 180 degrees? With the coolant level low collant willnot flow through the heater core vehicle will run hot If one of the hoses are warm and the other cold that run through to the heater core the heater core could be clogged, the thermastat controls the coolant to heat it up in the engine. I would check those theree things before we move on with the heater control unit inside the car.