Principles of Operation
WARNING: This page is about a different car, the 2013 Ford F-550 Super Duty, 2013 Ford F-450 Super Duty, 2013 Ford F-350 Super Duty, and 2013 Ford F-250 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
The Body Control Module (BCM) supplies the switched and control voltage to the horn relay (integrated into the BCM). When the driver air bag module is pressed, the horn switch (which is integrated into the driver air bag module) is closed and ground is supplied to the BCM which is routed through the clockspring and Steering Column Control Module (SCCM). The BCM then energizes the horn relay sending voltage to the horn, enabling the horn to sound.
The BCM provides a momentary ground to the horn relay when the anti-theft system is armed, and an on then off ground when the panic button is pressed on the Remote Keyless Entry (RKE) transmitter.