Just completed the install a la Don and O5Ultra. I used the plastic version of the horn, instead of the metal one, and it was very easy to cut the tunnel pieces away with a die grinder/cut off tool. A 90 degree 1/8 NPT X 1/4 carburetor nipple (from the hardware store) worked for the fitting on the horn end, and I used 1/4" vacuum tube for the connection. The little relay they give you with the horn will fit onto one of the two ground stud posts under the seat iin the compartment in front of the battery with a bit of reaming on the relay hole. Everything is zip tied in, including the horn piece under the OEM cowbell. Plug the original horn leads into the plus and minus switch posts on the relay, and run an inline fuse (I used 25 amps) from the battery to the relay, and then a short lead from the relay to the compressor (all of which fit in the space under the seat in front of the battery), and you're done. I used 14 gauge wire all around. Didn't cut any original wires, and it would take me about 20 minutes to put it back to OEM.
The install looks completely stock (except for a very short run of hose visible at the horn mount to connect up to the horn), but sure makes a big difference when you push the horn button.