When it came to changing my OEM horn I elected to leave the old horn in situ
and break into the loom about six inches ( 15cm ) outside the fuse box in the engine compartment.
The positive wire (Orange) now connects to the relay required to fit a slightly larger and much louder Stebel Nautilus. The negative side of the relay is earthed.
For convenience you can use the same earth point for the horn as well.
DO NOT attempt to use the green/yellow as it is a Canbus wire. (I did and had all sort of weird and wonderful things happening!)
The Nautilus is fixed with just one bolt and I have found a spare hole in the n/s in which to fix it.
The actual supply to the horn is taken via a 20 amp in-line fuse from the fuse box common positive which is a large gauge red cable. The extra wiring protected by some nylon 'cable wrap'.
The bumper is big and quite heavy and removing it by yourself is fraught with the risk of scratching its paintwork. You will need to unmount the high current fuse box (PSF?), air hose and the plastic plate this is
mounted to. Took me about an hour and I got all of the connectors and extra wiring from Hellfrauds. The Nautilus I bought from Hellfrauds but I can no longer see it on their website.
For an account of all the fun and games I had see this link: