Good morning Dan.
For your problem, I would start by examining the vaccum line that goes from the engine to the HVAC vaccum reservoir (it's a black sphere, probably on your passenger side, in the engine compartment - though some are inside the fenderwell).
Remove that hose from the sphere and ensure it is pulling vaccum, engine running. If not, find the hole or break in that hose. The fitting that hose attaches to at the sphere, is a check valve. Those sometimes go bad, letting vaccum decrease inside the sphere. The sphere itself could be cracked, as well.
If all that is good, open the glovebox, push the side in and open all the way and you may find a vaccum problem in there. Worst case, is the control itself, but I'd rule out a lot before I went after that being the problem.
How's your idle? If it fluctuates a lot, it could be a gross vaccum leak in the engine, PCV hose, intake tube loose, etc, making your HVAC control problem merely a symptom.
2000 2.5 MO5D 4.10 one-wheel drive DD - gave to my older son
1990 STX 87 2.9 going in.....Junked it
2000 3.0 4WD M05D 4.10 S-cab Off-Road package with 235-75-15" tires, hotrod DD, lol