What is this connector 68 Barracuda

Even though the harness connector fits the switch, the wiring from there on is quite a bit different. With heat only you can turn the heat on and turn the blower off.
You cant turn the a/c on and the blower off or the vaporater coil would ice over. So it wired in such a way that the fan switch is controlling a/c clutch i/o power.
That's why those who cant correct the wiring, change the controller and bezel, ad a toggle switch to get the blower to run.
I haven't studied the wiring daigrams lately but if my memory serves correct, the blower power is routed from the fuse box to the motor. The resistor bank and switch that provide the different fan speeds is actualy in the path to ground. If thats the case, A relay in the power supply wouldn't protect anything on the control side of the circuit.
The blower is one of the higher amp loads inside the cabin so a relay in the supply side could extend the life of the ignition switch, bulkhead connections, etc..