The temp sensor in the radiator and the A/C system both run the original fan in front of the condensor. I'd use the wires going to the original fan diode on the fan bracket to power the aftermarket fan as they are already wired through the water temp sensor, A/C system, and relays. The low pressure switch that was on top of the A/C drier will keep the fan from coming on with the nonexistent A/C. Possibly the A/C compressor button would turn on the fan if you jumped the low pressure switch.