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That helps a lot, so you do already have a relay harness. Those two wires labeled "back to stock horn harness" in the amazon ad, splice those onto one of the factory horn pigtails.
That splice to the factory harness will only be used to tell the relay when to close and draw power through the new harness and send it to the horns. The new battery cable will handle all the draw of the horns, and the old harness will draw <1 amp to activate the relay.
No need to go anywhere near your wheel or run anything up into the cabin.