The horn button sends a negative control signal from pin 8 of the clock spring D047 to the coil for relay T03 of the engine compartment junction unit B001 (pin 21 of connector A).When the relay is energized it closes the contact providing the power supply for the single tone horn P005 (pin 1). This power supply is sent along the line protected by fuse F10 of B001 (pin 27 of connector A). The horn P005 is earthed at pin 2.