Heater Cores

First check to see if the coolant is reaching the heater by removing both heater hose that go to the heater core. One hose feeds the heater core the other is the return, similar to a hot water heating system used in a house. You should have coolant in both. Next with the hoses removed try to run water through the heater core with a garden hose. If the water flows freely and coolant is present in the hoses, I would then check the the valve that controls the flow to the heater core to see if it is working properly. Good luck Backyardmechanic