1995 GMC Jimmy vents don't work

Suspect vacuum leak. Check your vacuum lines to the vent doors under the dash, usually in the passenger footwell. There is a door that opens and closes to direct heat to where you tell it to go using the dash control. Also check for free movement of the door. If those all look ok, you probably have a bad dash control but vacuum leak is more typical.

Defrost is the fail safe mode. Look for a vacuum leak under the dash as bluelightnin said, Also look for one under the hood. The common ones are a split hose off the vacuum tee at the back of the engine, The connection to the plastic hose where it has the rubber pieces pull apart. Also check for a leak at the vacuum booster and the vacuum ball under the hood. With the mileage and the year I would just replace any of the rubber vacuum lines and check the plastic ones.