If its the one I think it is, probably yellow, then it goes to the coil, and you are going to have to figure out a way to adapt it.
When you are cranking, the battery voltage drops due to the load. So, in order to make sure you have a hot spark, coils were designed to operate at a lower than 12 volt voltage.
A resistor was placed in the run circuit to drop the voltage down to its normal running voltage when you were running.
That wire, which is only hot when you are cranking, bypasses the resistor and lets full battery voltage go to the coil just when the starter is drawing a load.
In this case, if I am right as to what wire it is, you have two choices.
1 tape the wire off and run the truck without it. With the decreased load from that starter, it may not be necessary any more.
2 if it is necessary, you will need to add a Ford starter solenoid to the circuit. It can fire the starter, and it has a terminal for that purpose that you could hook the wire to.
Someone else should chime in here shortly who has done one and can give some real world examples.
On my old Ford tractor, when I converted it to 12 volts from 6, I added a resistor to drop the 14.5 from the alternator to 6 going to the coil.
But, I put a momentary contact switch on the dash that would bypass the resistor and put full battery voltage to the coil briefly.
On a real cold morning, or if the battery is weak for some reason, I can bump that switch while cranking to give it an extra hot spark.