Have you checked the wires/connectors on the horn itself? The horn on our micra didn't work, replaced the fuse (still didn't work) and listened for the relay 'click' sound when pressing the horn, which it did so knew the relay was ok. Checked the horn itself/wires, noticed the wires had snapped where they go in to the connectors! Replaced both connectors with 5amp spade connectors from halfords and it sorted it!