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 agree.
Then to carry it farther, if the control valve is the issue, you should be able to remove the vacuum from the control and have air coming out of the defroster ducts, then all you need is temp control. I seldom switch off from defrost during the winter, and normally control the inside temp by fan speed, but usually my winter trips are not very long.
For the record, without the AC, the defrost isn't nearly as quick clearing the windshield. The AC removes the moisture from the air that hits the glass, that dry air clears the glass 2x as fast. That was the main reason AC became common on vehicles, faster clearing of the windshield in defrost mode. I believe that the Government required (or mandated) AC to be included at one point in time.
