The factory heater valve is known for not closing completely and the constant leak of hot coolant works against air-conditioning in summer. Engine coolant heater coils and air-conditioning refrigerant coils are a single unit.
When a underdash coolant rubber hose breaks, and they do, it very often dumps steaming coolant on the driver, passenger and or fogs up the windshield. Obviously when this happens there is a great potential for calamity. This is a well-known problem and tech inspectors will almost always require manual shut off valves, located in the engine bay, for any Pantera participating in a track event.
So to solve both of these issues many owners installed two manual shut off valves in the engine bay, resulting in pleased track event inspectors and better functioning air-conditioning.
But there are days in the spring and the fall when an early morning drive needs a heater but late afternoon calls for an air conditioner and that meant stopping and crawling under the car to open and/or close the manual valves, assuming you had mounted them somewhere where you could reach them.
Enter the four port electric heater valve and I think you understand why those of us with the manual valves saw the electric valve as a worthwhile upgrade allowing simple cockpit opening and closing of the heater core coolant.
The dash control is a potentiometer that opens and closes the valve in increments
It is likely that your under dash heater valve has been deleted or is in a non-operating condition. To still have it functional - or even remaining in the piping - seems redundant with the electric valve also installed.
Mine was removed during collision repairs and will not be reinstalled as I am going with a new aftermarket compact HVAC unit from Restomod that incorporates their own two port electric heater valve.