Heat is stuck on windshield setting

[Firexit1A]

The heat in my 97 4wd Ranger is stuck on the defrost setting. Reguardless of where the dial is nothing changes

 

[RonD]

You have lost Vacuum for the dash board vents.
Defrost setting is the default position for all vents, so OFF or No Vacuum = Defrost

Not an uncommon problem.
On passenger side of engine bay, coming out of the firewall/heater box you will find 1 or 2 hard plastic Vacuum lines, they can look like wires, they are not, there will be a Black and Grey vacuum hose.

The Black one is the one that has broken or the Vacuum reservoir has.
It often gets too close to exhaust and melts or gets too hard from heat and breaks, any breaks or melted parts can be cut out and replaced with rubber vacuum hose that fits snugly over the hard vacuum line.

The lines runs to lower front of engine bay to the vacuum "ball"/reservoir
Picture here of possible location: http://www.explorerforum.com/forums/index.php?attachments/vacuum-globe-jpg.39621/

So hard to see under inner fender in front corner.
The "ball" can crack and not hold vacuum.

If you have done any work inside the cab then you may have disconnected a vacuum line under the dash, main line comes in passenger side up under the glove box