Well it's not my prettiest work, and yes I should have painted them first, but here's what you can do when you just happen to have a pair of GM W body (Monte Carlo, Grand Prix, Olds Cutlass) horns laying around. I installed a power distribution block, relay and ran new wiring to the horn switch, then used the existing wiring (just the pink) out to the horns. The only thing that stumped me was why I had to isolate both horns from the frame of the bike to get them to work properly. But now they're installed and holy crap they're loud! They were $15 for the pair at my favorite bone yard, then from Amazon the distribution block was $6.50 (one 4 gauge in, four 8 gauge out), a 5 pack kit of relays with harnesses was $14, and a 5 pack kit of ATC fuse holders for $12. And the best part is there was plenty of room to mount them behind the forks, so there's no clearance issues at all.
Nice! I wish I had that kind of honk under my thumb switch.....