Circuit/System Description
WARNING: This page is about a different car, the 2022 Cadillac CT5. However, it is still accessible from the selected car via links, so may be relevant.
The body control module (BCM) controls the horn relay by providing a ground to the control circuit of the horn relay coil energizing the relay. When the horn relay is energized, the horn relay contacts close applying voltage through the horn fuse and the horn control circuit to the horns.