We've found it necessary to add an extra horn to our 2002 Insight due to the size differential (and lack of hearing) between Insights and SUV's, which seem to lumber along here in Michigan in massive quantities with their sound systems set to 120dB+ SPL.
So, some research time was spent figuring out the best way, mechancially for mounting and electrically for safety, to do the job.
This is what was found:
1. For mounting, the aluminum strengthening bar that runs parallel to the hood near the firewall has some extra holes already drilled in it so a Threadsert was installed in it to provide a strong method of mounting without compromising the integrity of the member.
2. A cadmium plated bolt and washers was used to install the horn as this type of plating is fairly corrosion resistant, although stainless could also be easily used used. Be sure to use an internal/external tooth grounding washer for the horn to ground to.
3. The horn used was a single 118dB unit purchased at Murrays for approx. $10 and in conjunction with the stock horn, the volume is quite alarming although a set of small air horns could have been used but on second consideration was decided against due to the time lag for the pump to build up pressure, as well as introducing another moving part into the maintainence picture.
4. Anytime a new electrical "add-on" is introduced into a vehicle, two words to live by are: "Relay and Fuse".
5. On the drivers side under the hood near the front of the vehicle is a small fuse box that houses the horn relay and fuse. Now the reason that is mentioned is only to note you'll find the relay trigger wire contained there-in. One must lift out the radiator overflow tank (it pulls straight up), and carefully remove the 2-10mm bolts that hold the large fuse box in place. Be careful not to drop those bolts, they're a bugger to fish out.
6. Then flip over the fuse box and look for a single orange (with no tracer color wire, approx 20 guage). This is the switched ground from the steering wheel that activates the horn relay. Use this for the switched negative on the relay you're putting in.
7. Ensure that the +12V supply for the relay comes from the battery and is FUSED with at least a 20A fuse. If possible, try to use one of the newer plastic blade style of fuses and holders.
8. Be sure to securely seal the connections on the relay with electrical tape or some other electrical sealer to avoid water intrusion and corrosion.
9. Neatly tie-wrap the wiring in place along the back harness near the aluminum strengthening bar.
10. TOOT-TOOT, BEEP-BEEP