As a stand alone heater- very simple as the ac and the heater are actually separate inside thebox. Consider it like a duplex house. One family on one side and the other family on the second side but they share the same front door.
Now if the box controls are vacuum then you will need a service manual for the car to help the connection of the doors to open when you slide the controls on the dash. Typically one vacuum line from the engine goes inside the car to supply vacuum to the controller and it basically opens a valve like device (not actual valve - I’m just trying to keep it simple for this post sake) to move a pod that actuates the doors inside the unit.
If you had Just a heater box (no ac) the doors are actuated with cables. Also most cars the ac box fire wall is different than the just heat firewall. So swapping out the boxes is not as easy as it seems.
Syleng1
