Since you've checked all the system components, it's got to be the wiring.
Check you're getting 12V at the horn wire connector when the button is depressed. Check the horn grounding wire at the horn. Clean any electrical connectors in the circuit and the horn chassis ground.
If that fails, you're looking for a wiring break in the orange/blue (12V power) or black (ground) wire. It's a simple circuit and problems should be self evident.