If there is air moving when the blower is running, that resolves a couple of questions.
If both heater hoses to/from the core are hot, that resolves another couple of questions.
That really only leaves the blend door/dampers, and the controls for that. The temperature control in the dash is really only a potentiometer for most of that vintage GM vehicle.
https://www.youtube.com/watch?v=Gxxe7ZKfeIQ