I dunno about trucks, but cars of that era would default to defrost if vacuum was lost to the controls.
I would pull the line off the vacuum can that controls defrost and see if you are getting constant vacuum there, if so the control unit is probably the issue.
I was just gonna suggest that.
That said I have a project in the garage, an 85 d150.part of the project includes adding "factory" ac to a truck that originally came w/o
I have most of 3 factory ac heater boxes to scrounge and cherry pick the best components from to (hopefully) build up 2 "factory ac" boxes among the pile.
I say most o, because one housing is busted all to hell probably from rough removal) not done by me, was a swap meet find as it sits)
My main concern is can anyone tell me if the gaskets and seals within are available anywhere, whether nos or possibly from one of the aftermarket rubber/weatherstrip places for these trucks?
