For complete circuit diagrams, refer to Horn/Cigar Lighter in the Contents of Group 8W - Wiring Diagrams.
(1) Measure the resistance between the horn(s) mounting bracket(s) and a good ground. There should be no measurable resistance. If OK, go to Step 2. If not OK, clean and tighten the horn mounting hardware as required.
(2) Disconnect the wire harness connector(s) from the horn connector receptacle(s). Check for battery voltage at the horn relay output circuit cavity of the horn(s) wire harness connector(s). There should be zero volts. If OK, go to Step 3. If not OK, repair the shorted horn relay output circuit or replace the faulty horn relay as required.
(3) Depress the horn switch. There should now be battery voltage at the horn relay output circuit cavity of the horn(s) wire harness connector(s). If OK, replace the faulty horn(s). If not OK, repair the open horn relay output circuit to the horn relay as required.