Typical Cooling Fan Relay

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The "30" terminal connects the relay to the battery or 12V source.
The "86" terminal connects to the master arming switch - either a separate switch or the ignition switch.
The "87" terminal connects to the fan.  The "87a" terminal can be left empty since it goes to ground.
The "85" terminal connects to the accessory trigger (thermostat activated) switch.

The in-line fuse capacity should be above the amperage pull of the fan motor itself.