Heater control unit change out.

[eighteenninetytwo]

I am changign my old control unit with AC for a new one without as I dont have an A/C box anymore. whoever rewired the truck kept a 5 pin connector on the control unit and the new one has only three. In short I expect the wire from the fusebox to go to one pin and then to the fan motor directly do I figure out WHICH of the three pins it should go to (on the 5 pins connector it went into a middle pin which doesn't connect now).