there is an electric motor under the radio above the transmission hump. It operates a small door inside the air duct that mixes outside air and heater core air to get the temp you want. First thing to do is pull the **** off the dash lever and see if the **** is tight on the shaft, or slipping. If it is turning, lay on the driver side floor and look up toward the radio. That is the motor. Take the plastic panel below the radio and above the hump off and look at what moves while you stroke the contnroll **** from hot to cold. Can be motor, small gear box, or cam level tied to the door.