Sounds like you have an Auxiliary Heater. The knob you describe sounds like the control for the auxiliary heater. It can be turned to act as a timer to preheat the vehicle if you push it and turn it. Mine is about a 15 minute timer. How far you turn it when not pushing it in sets the temperature (it acts like a thermostat). If the vehicle is not running it will turn itself off when the time runs out. When the vehicle is running it will supplement your heat from the engine when it is turned on. This, of course, is all true if the auxiliary heater is properly working.
 
Marve
'82 diesel Vanagon

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