I've got a Stebel Nautilus, Aerostich calls it an Earcannon, that is pretty much the same as your horn. It came with a relay, 20 amp if I remember right. Anyway, a couple of positaps, wired the relay in parallel with the stock horn, hooked the air horn to the relay, no worries, took about 20 minutes.
I mounted the relay right behind the stock horn, drilled a hole in the frame tab behind the front fusebox and mounte the horn there.
john