Defrost quit after engine change, blower motor still blows, but not on windshield! What's wrong with defrost? 1988 S-10

On this year vehicle the flapper should be vacuum controlled. You may want to look at the vacuum lines going to and from the vacuum canister in the engine compartment. If it will change from floor to dash but not defrost then you may have a bad vacuum line or diaphragm in the dash from the selector.

Rear Defroster

This vehicle was also equipped with a rear defrost on some of the higher end models. The rear defrost is completely electrical. The power runs through a fuse and then to a relay that is activated by the rear defrost button on the dash. Then the power runs to the rear window strut. The rear defroster diagram to your right may help if you need to trace down a wire or two.

Blower Motor Resistor Location