The idea of using a relay generally is, to use it when the device you want to turn on requires more current than powering it directly can handle. This is why signal wires to a relay are so small, and the switch side uses bigger wires. Best example of this on a car is the blower motor.
Looking at the wiring diagrams, I just noticed it actually grounds the wire at the horn, so you would need a relay to convert the power signal to a ground. You'll need a 20A 4 pole relay that is normally open. Try this:
The wiring can all be accessed in the passenger footwell area. Horn relay at the continence center, and the power lock relays are in the same area.
[This message has been edited by RWDPLZ (edited 07-08-2014).]