Have engine heated up and heat set to HOT, shut off engine.
Open the hood and look at the firewall on passenger side, you will see two hoses.
Feel them, one should be warmer than the other but BOTH should be hot.
If they are not hot then read on, if they are hot then you have a broken Blend Door, Google: 1997 Ranger Blend Door
If hoses are not hot:
Follow those hoses to the heater control valve or By-pass valve
By-pass valve will have 4 hoses connected
Control valve just two hoses
There should be a vacuum hose on either, usually grey in color
Follow the vacuum hose, it will run to the firewall and go thru it with a Black vacuum hose next to it.
Black one controls your vents, i.e. defrost, panel, floor
Make sure there are no cracks or breaks in the grey vacuum line, OR the Black one!
Start engine, manually push the arm on the valve to open it, hoses at firewall should start to heat up.
If not replace valve/by-pass
If hoses heat up then release arm and pull off vacuum hose, put finger over it, it heat is set to HOT then it should have a pretty good vacuum sucking on your finger, if so replace valve/by-pass.
If no vacuum and hose was good then you probably have no vent control either, which means vacuum reservoir is most likey problem.