The low speed trigger wire on pin 42 needs to run to pin 42 and the green line on the green a/c connector that way the fans run on slow at operating temp or when the a/c is on. The high speed relay would be triggered by pin 33 only and you would get high speed fans only when above the trigger value programmed in the PCM. You could also jumper another lead on the high speed relay to run into the cab so that you can manually turn the fans on in stop and go traffic.

You may have to wire a diode into the individual ground circuits to prevent false feedbacks from other grounds but that's simple enough. A diode is a one-way check valve in the electronics world, allows signal to travel 1 direction only.