That's how my horn is set up. You could wire it with a relay, but since it's only used occasionally for short bursts, I wired mine directly.
First connect a piece of wire to the + battery terminal, 16 gauge wire should be plenty. Then run the wire into the cab, there are a few different holes in the fire wall to choose from. Be sure to put a fuse in the wire, the closer to the battery the better. Once your inside the cab decide where the button will go and connect the wire to it (there will be 2 terminals, it doesn't matter which one you use). Then run another wire from the other terminal on the button back through the firewall and into the engine compartment. The stock horns (if they still work) are behind the drivers side headlight. There is only one wire going to them, it should be blue. If you just cut that wire and use a butt connector (assuming you don't know how to solder) then the stock horns will be powered by your new button.
Note: be sure to completely disconnect the battery before you begin.
