ive replaced the thermostat.flused the rad and the heater core and add new coolant
my heater only blows warm or cold air on 1996 Dodge Ram 1500
You may be having a problem with the temperature door. It directs airflow though/around the heater core. If the door is not operating properly you will never have warm air from the heater. I would check the fault codes in the A/C control system, and physically inspect the actuator for the door, make sure it is connected and moving when you turn it to full heat or cold.