Fan motor power is three different resistances to ground via a three-position switch, which is exactly like the Mustang’s original switch. Where it differs is air-conditioning compressor clutch power via a micro switch activated by the factory heater control. The micro switch is closed mechanically, which gets power to the thermostatic switch at the evaporator. If temperature warrants, power then travels to the high/low pressure switch. If the high/low pressure switch is closed, the compressor will engage. Power must be switched and only when the ignition switch is on. It must also be fused.