I have a 1986 Safari with the same problem. In my van, the heater unit is located behind the drivers seat and is attached to the outside wall. The heater core is inside this box.
When operating the dash controls for heat, vacuum is applied to a valve located on the passenger side of the engine compartment. It is down very low and can only be seen from underneath. (There are two fluid hoses in this area for the coolant).
I suspect the valve is seized in my vehicle. Due to the vans age, I have not tried to fix it.