Found this wiring diagram on the link you posted:



Because of the high amperage draw of the new horn, the manufacturer wants you to use a simple relay to switch a new independant circuit feed to the horn. The relay gets its switch signal from your existing horn circuit. This is really the best way to do the wiring and is what i would recommend. Raising the value of the over-current protection (fuse) on the existing circuit can compromise your wiring.