I just received the Stebel Nautilis and installed it this last weekend. Had to whip up a custom harness for it to get the relay in there, but I left the stock horn on there as well and just adjusted it up a bit. Custom harness wasn't hard, but took a while and I would definately go with a PNP next time. I just have this thing about doing it myself at least once to get the experience. Like many others, I mounted it on the right side just under the upper and mid plastics. I hated putting it there because the openings for blasting are pointed at the plastics, not at the idiots in front of me, but it is loud enough to compensate.
Only had to use it once so far, but my comment on that is that when I am reacting to an *idiot* I am not thinking about a horn, I am finding my escape route and making sure I'm safe. After all is well, THAT is when I communicate with the offending driver. I must say, the eyes appeared to double in diameter on her when I laid on that thing. Good investment for about $45 total.
Documentation says 138db @ 4 inches, more like 118db at 4 feet. It's still an attention getter though.
-SparX