It just so happens that the tail light wire is very easy to access and tap into under the seat to trigger a relay. It is right next to the battery as well, so the relay's the load can be taken from the battery positive easily and with a short run of wire. Also, being under the seat, its an obvious waterproof location for a relay.

The side/tail light circuit is only powered up when the ignition is on (or the key is in the "P" position), so provides an ideal switched supply. As the sidelight circuit runs pretty much all round the bike, you can tap into it just about anywhere that's convenient for whatever you're doing.

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