I installed a twin tone, 139 decibel air horn under the hood, right near the front grill, facing outward. Used in conjunction with the stock horn, this thing is really LOUD! The horn was wired through a relay, installed under the hood, and still runs off of 12v power. I put a red illuminated switch, just to the left of the shifter, and I wired it to stay illuminated while the car is running.
The horn is Stebel Nautilus Compact Mini Air Horn.
Wiring Instructions:
Pinout: 
Everyone needs spinners on their subwoofer, right? This sub hits a lot harder than most imagine, due to it’ efficient design. It bounces the audio off the back of the car, allowing a longer wavelength in the cabin, producing more low end without the added weight. #audionerd
I later added a three-way switch (on-off-on constant) for the grill for car shows, etc. 
First up, for me, is fixing the factory audio system. I have never known much about cars, but I have always installed third party audio systems. I am a sound engineer, so I’m very picky about audio, and have very specific audio needs. It is often important to check mixes in the car, and compare with other audio reference material. I did a bit of analysis with the audio, both with a pink noise generator and frequency analyzer and my old school ears. The factory limiter in the audio path sucks, but it can work with proper gain staging and third party solutions.
Here is the trunk installation. These are the wires running along the driver side, under the doors. They run into the two fuse blocks, and the amp power goes directly to the battery (fused.)
The wires were brought into the back seat, then sent up to the front, where they were spliced together with the factory speaker cable. This line includes direct power for the power amplifier, remote 12v switching cable, subwoofer remote control, & speaker sends. The large, heavy stuff was all mounted to the top of the trunk, under the back window. I used self tapping screws and made sure everything is very tight and solid, as the subwoofer will be vibrating right near there. In the picture, you can see the power amplifier, audio crossover & Speaker to line level converter.
I tucked away the wires as tightly as possible, as my children can be Ultra-Massive-Destructoids, and anything hanging, dangling, or lying about is subject to their wrath. I didn’t want any chance of having to do this again because of carelessness, or because a Nerf gun battle broke out and the trunk was used for cover…
I used a distribution block for power, since the amplifier and the crossover had different power requirements. The crossover has a remote control function – this is used for the subwoofer, so that may be adjusted on the fly, without use of the radio or other controls. 
Subwoofer Control – lines run along door panels, and up underneath steering wheel:
After the full installation, I was rockin’ for about two weeks – then all of a sudden, no bass! Uh oh – the amplifier i had used was from a previous installation, so it already had a bunch of hours on it. Needless to say, the power transformer blew on it – It wasn’t worth the time for me to fix it, due to it’s power and age, so I replaced it with a Kicker DX.500. This is a mono, 500 watt power amplifier specifically designed for this purpose. I added a 0/1 Awg ground line for power in the trunk when I replaced the unit.
From the driver seat, I have control of the EQ and subwoofer controls, and from the trunk, I can alter the crossover frequency, bass boost & output gain. The Polk speakers compliment the Kicker 3.5″ in the front dash, as the Kicker full range 6×9 speakers are a bit too “hyped” for my taste. In other words, they boost the high frequencies in our vehicles 3-5 dB at 8K and higher. The Polks have always sounded smoother to me, and the addition on the subwoofer and dash speakers give it a huge sound – not a flat frequency curve, but a pleasantly pleasing one.
Subwoofer in my trunk (that’s an emergency tie-down strap on the right) It was later painted and modified.
