[yorkie_chris]

No no no.... never replace a loom ... just carve it up and replace the rotten bit's untill you get that annoyed with it you rewire the lot, that's what happened to mine anyway!

Right then wiring 101... #searches for wiring diagram... only got 10 or so copies...#

The horn is fed from the signal fuse, it gets a direct feed from the loom, so firstly check the orange/green wire that leads to the horn. Does it have voltage? It should have voltage any time the ignition is on.

The next is the earth, which is a switched earth for some reason. Follow the black/blue wire up to the connector for the LHS switchgear, it should have continuity.

Then, the switch itself, short out the black/blue wire and the black/white wire on the LOOM side of that connector, it should sound the horn when the ignition is on. If it does, then it's the horn button at fault.

Then it flows to earth via the black/white wire, this should have continuity to earth.