Check the fuse first..I think it is number 27 in interior fuse panel. Check coolant level and make sure it is full. If low, hot coolant may not get to the heater core. If level ok, with car warmed up and running, put your hands on both heater hoses going to heater core. Both should feel hot, the same temp. If one isn't hot then the heater core is probably plugged. If both are cold, the problem is the water pump or cooling system. Otherwise, pull glove box out....behind the door is the blend door motor. The motor has a shaft that turns the door. It will turn about 1/4 around as you switch from hot to cold. So with truck on move knob from hot to cold and see if the shaft turns 1/4 around. If not, the motor is bad.