The heater control valve is right in the center rear of the engine compartment on the firewall. It is black plastic with two hoses going to it and a cable. Get a friend to move the heater control lever from max cold to max hot and see that the valve operates through its full range. The engine doesn't have to be on to test it this way. Start the engine get warm operate to the Hot position both hoses should feel hot. Is your Thermostat OK?