Principles of Operation
WARNING: This page is about a different car, the 2004 Lincoln LS. However, it is still accessible from the selected car via links, so may be relevant.
When the horn switch is pressed, a message is sent from the instrument cluster over the network communication link to the FEM. The FEM acknowledges the message and energizes the horn relay by grounding the horn relay coil which allows the dual-note horn to sound.