Before beginning electrical diagnosis, check the fuses. Disconnect the electrical connector from the horn. To test the horn(s), connect battery voltage to the horn terminals using jumper wires; if either horn doesn't sound, replace it. If the horn does sound, check for voltage at the horn connector when the horn switch is depressed. If there's voltage at the connector, check for a bad ground at the horn. If there's no voltage at the horn, check the 
Relay. If the relay is functioning, check for voltage to the relay power and control circuits; if either circuit is not receiving voltage, inspect the wiring between the relay and the fuse panel. If both relay circuits are receiving voltage, depress the horn switch and check the circuit from the relay to the horn switch for continuity to ground. If there's no continuity, check for an open circuit; if there's no open circuit, replace the horn switch. If there's continuity to ground through the horn switch, check for an open or short in the circuit from the relay to the switch. For replacement on 2006 and earlier models, remove the left front turn signal/parking light housing; for models with two horns, the bumper cover must first be removed for access to the right side horn. For 2007 and later models, disconnect the electrical connectors and remove the bracket bolts. Installation is the reverse of removal.