[Barnsnake]

It's a black wire (#953) in the turn signal section and a green wire (#924) in the headlight section. The other side of the horn goes to ground. This assumes you have the Painless panel with a horn relay. The horn button, when pressed, grounds the circuit and closes the relay which supplies power to the green wire.

You can download a PDF of the Painless manual and diagrams from their web site.