As others have said the parking light circuit and you don't really need a relay.
A pair of spots at 10 Watts each is only going to draw 2 Amps and the lights typically operate at full brightness from 9 - 30 Volts
In other words you can afford to lose 3 Volts, at 2 Amps you shouldn't lose half that.
On my V7III I have 2 spots fed through 2 Amp reed switches so I can turn them off by removing a small magnet stuck to the outside of the headlight bucket.
Horns on the other hand do need a relay if you want them to be loud need to be mounted on the spring steel bracket they come with.
The small Blue/Black pair of wires are also from across the park light and go to the battery Voltmeter.
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If you are still going to use a relay power the coil up from the park light (yellow wire), you don't need to add a fuse there but use a 5 Amp fuse to supply the 30 terminal, there is lots of room in the headlight bucket for the relay and fuse and you don't have to cut into the main loom.
https://www.thisoldtractor.com/guzzi007/schematics/2000_Jackal.gifNote how the red wire to the ignition switch and the brown wire back to the fuse block is completely unprotected, this is very typical of Guzzi looms, I usually add a large fuse e.g. 40 Amps at the battery end.