The Heater control controls the heat setting using the Blend Door Stepper motor.
The Vent control is done by vacuum...if vacuum from the intake manifold is lost going into the cabin then the air only comes out of the demister vents. I had this happen to me on my CRD after a service at the Dealers...after picking up the jeep I noticed that air was only coming out of the demister vents...I went straight back to the Dealers and they explained that they had knocked off the vacuum pipe from the vacuum pump that the CRD uses.
The Service Manual for the KJ explains that the Blend Door motor moves a flap to control the airflow over the Heater Core to adjust the heat...no water valves are used on the Heater Core.
Then there are two Mode Door Actuators alongside the HVAC...operated by vacuum.
One Mode Door Actuator controls panel vents while the other controls the floor/defrost doors. So there is a vacuum line coming into the HVAC from the intake manifold and then vacuum lines connecting the Mode Door Actuators to the relevant vents...one of these vacuum lines may have been knocked off.
There is also a Recirculation Door Actuator to close off outside air...this is also vacuum operated.
Download the KJ Service manual for the relevant year KJ here and have a look at "Heating and Air Conditioning" Section:
Index of /manuals/Jeep/KJ