97 Bravada stuck on Defrost

I am working on Father-In-Laws Bravada and the heat is low in temp and I will look at the Coolant level and Flushing that. But the real problem is the fact that it is stuck on defrost. Now matter what I have the dial aimed at it just comes through defrost. Any suggestions?

You've lost the vacuum supply to the heater controls. Mine was a broken connection on the vacuum "ball" tank mounted on the underside of the hood.