Principles of Operation
WARNING: This page is about a different car, the 2006 Mercury Milan, 2006 Lincoln Zephyr, and 2006 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.
The battery junction box (BJB) supplies the control and switched voltage to the horn relay (part of the smart junction box [SJB]). When the horn switches are pressed, ground is supplied through the steering column to the horn relay. The horn relay is then energized directing voltage to the horn and enabling the horn to sound. When the vehicle security system is armed, the SJB provides a momentary ground to the horn relay control side to indicate the system is armed, and an on and off ground when an intrusion is detected or the panic alarm is activated.