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Since the horn failed I'm assuming it was going off constantly rather than on and off like the alarm would produce?
The horn can be controlled in two ways. The body control module (BCM) can control the horn relay for alarm and keyless entry horn chirp functions, or it can be controlled by the horn switch.
From the horn switch or BCM a signal goes to the horn relay. The horn relay then supplies power to the horn.
You may have a stuck horn relay, but if the horn started going off when the car was parked then that isn't likely. You can still swap the relay though and see if that helps.
It could be the horn switch itself or a short to ground in the wiring anywhere between the switch and horn relay.
It could also be an internal problem in the BCM.
You should have two horns which are mounted right behind the bumper beam on either side.
Is the BCM located in the steering wheel air bag or in the steering column?