The horn switch is the mechanism that completes the electrical circuit to sound the horn. In most vehicles, this is a membrane switch or contact plate located inside the steering wheel hub, under the airbag assembly. When you press the center of the wheel, the switch grounds the horn relay, sending power to the horn. In older vehicles, it might be a separate button. If the switch fails (often due to a broken “clock spring” cable in the steering column), the horn will not work, which is a safety hazard.