My fan has wires that run from each fan to dual 30amp relays. I have two 30amp fuses one each relay pin 30. The switch power side of the relays gets jumped off of the same wires as the power feed ( split after the fuse ) to pin 86. Pin 87 goes to the fan motors. Pin 85 ( relay switch ground ) goes to a manual switch that's grounded under the dash. I've also used it to switch using factory style grounding fan temp switches. Both of my fans motors are grounded at the battery.
I don't like running 12v live wires into my interior. Triggering an accessory using the ground side of a relay is a little safer. Most manufacturers use pull down circuits.... meaning they switch with the ground side.