Could be several things,, Check these two first: First: get the truck plenty warm use heater while running the motor, then turn off motor and feel both hoses going from the motor to the heater core, Are they equally hot?, If not it is likely that the heater core is stopped up. SECOND: verrify that the controlls are working correctly, best way to do this, Start motor let it idle, lean down into floor and slowly move the controlls one at A time listening for A change in noise of air and also for any doors to operate.