Does anyone know how to make the stock 1980 Yota heater work with an S-10 wiring harness?

 

I would think that just the main power wire going to the heater was somehow disconnected in the process of putting the S10 wiring harness in, meaning that the wire just needs to be attached to power. I am guessing that you have a 4.3 V6 swapped in your truck. Look at a wiring diagram for the heater and see what wires are attached properly and which ones are not. You can get wiring diagrams at www.autozone.com.

If the wiring is completely taken out of the truck, you will have to make something custom. If you do not care about having different blower motor speeds, just wire a switch to a relay that controls the blower motor. All the other heater controls are attached with cables.