Adding a sensor to the horn wire.

I would find one of the horns under the hood, and tap into that with a relay. You won't find anything inside the car. The insane engineers have seen fit to involve two computers in blowing the horn. In the past, the horn switch turned on the ten-dollar relay, which powered the horns. The circuit was extremely simple and reliable.

Now, when you press your horn button, a varying voltage is sent to the most complicated computer on the car, the instrument cluster. That interprets the voltage as "horn request", then sends a digital signal to the "FEM, Front Electronic Module. That computer interprets the signal, then turns on the horn relay. The typical repair on Fords with a dead horn is around $800.00, so you don't want to be doing anything that could damage one of those computers.

Another alternative is to just add a pair of horns for the system you're adding. That way there will be no argument if you have horn trouble while the car is under warranty.