The gauge has its own power from the ACC position of ign switch.
For fan power, naturally, you are going to use the switch for controlling the fan relay.
And in making the job easier, as in running one wire vice two, wire toggle switch, or rocker, your choice, on the ground side of relay coil.
Run power to relay coil, then other coil lead to new switch and other side of switch to ground.
Now you are completing the circuit by providing a ground.
If you wire on power side, you have to run two wires, one from power source to switch, then back out to relay coil.
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