May sound strange but there's a "switched" (ie: ignition has to be on) wire on the positive side of the horn. The horn button itself controls the negative or ground side of the circuit.
With that said, the ONLY thing I'd consider running off of that wire would be the energizing circuit side of a 30 amp relay. Since all that takes up room I chose to mount a "switched" relay at the rear of the bike and use the tail light circuit to control the relay. There's more room for fuses or one of those neat (but pricey) Centech fuse blocks.
BTW.....
to the site - lots of good folk and information here.