Relay Technical Note
A relay is an electrical device which controls a switch. You can control the voltage at the output by using the existing positive wire to a device such as a headlamp or horn. In some bikes, the wiring harness will not support an additional device, but even if it will, adding additional current draw to the harness will reduce the voltage in existing devices on the circuit.
It should be used to trigger running lights from an existing headlight connection, or to install a horn if not already present. It can also be used to trigger an accessory from the ignition accessory setting.
Choose a relay rated for (at least) the voltage and wattage of the device you want to power.
You can install a relay by using spade connectors on the relay and wire taps, or by soldering the wires, protected by shrink wrap to the wires directly after cutting them. As usual, if you don't have this skill, consult a professional bike mechanic instead of cutting harness wires.
Relay connections are usually numbered:
30 is connected to the battery positive terminal via a suitable fuse
Connect the existing device positive and negative to 85 and 86
Connect battery negative and the 87 connection to the device you want to power. If you want the opposite of this voltage, and there is an 87A connection, you can use that.
Your bike model users forum or your shop manual will usually reveal the harness wires you need to use.