Purchased the Stebel Nautilus air horn (compact version) Saturday, here locally in Victoria (more on this below). Have not yet installed it and, Coho, I too would be curious as to installation, hints etc.
I did investigate this item on the web and, initially, came across two sources in the U.S. that, in addition to the horn itself, also supply mounting hardware specific to the Miata.
The first: <http://www.3rdstrategy.com/3rdMillennium%20Air%20Horn%20Kit.htm> helpfully posted instructions, including detailed pictures of the mounting hardware, <http://www.3rdstrategy.com/horn_installation_guide.htm>, which gives you a good idea of what is required (essentially a grounding wire about 6” in length with appropriate connectors and a bracket, a short piece of aluminum bar about 2” in length with two holes drilled in it). This particular kit is specific to the NA (‘90-07) only. Cost for the horn with the additional Miata specific hardware is $58 plus about $8.50 shipping from the U.S. all in U.S. currency.
The other source, also in the U.S., is <http://www.thompson-automotive.com/Air_Horn.html> The cost is lower ($41.50 U.S.) plus $10 shipping to Canada and also includes an installation kit, in this case separate ones for each of the three Miata models. As well, their kit apparently includes a 20 amp fuse that you exchange for the 15 amp one in your fuse box. No instructions are posted on their website but the kit is supposed to come with instructions. I suspect that other than the addition of the 20 amp fuse the kit is probably similar to the 3rd strategy one.
I decided to check to see if the horn was available in Canada, perhaps even locally. Lo and behold through Google I discovered that it was available through Scooter Underground <http://scooterunderground.ca/index.htm> here in Victoria (click on parts and accessories and then “horns” under “stuff for your scooter”) for $43 (Cdn). They are located on the north-west corner of Yates and Cook in the basement area of, coincidently, Pacific Mazda. Anyways, went down there and asked if they would offer a discount to VI Club Miata members and, after checking with the manager (name of John I believe) they said, yes – 10%. Furthermore, the discount applies to all their scooter accessories, including clothing. (Actually, some of the goggles look pretty neat, perfect for those who are really into top down motoring – thinking here of those brave souls who participated top down in the Xmas lights cruise in the middle of that snow storm Dec 20!) After my purchase ($43.35 including all taxes) I went upstairs to the Pacific Mazda parts department and picked up a 20 amp fuse (free – “my present for the day”).
If I have time on Sunday I may go to Home Depot to see if they have something that can be used as or fashioned into a bracket. The ground wire looks straightforward enough. But if anyone else has suggestions as to what can be used as a suitable bracket…
Finally, there is a fairly good discussion of the horn and experiences in installing it (but no detailed instructions unfortunately) on Miata.net
http://www.miata.net/products/pmisc/stebel.html