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This is from another site on the internet that I found, looks pretty handy

I purchased an air horn kit for my truck from Air Horns of Texas a couple of weeks ago (whom I would not buy from again) and finally bought everything I needed to install the system. The "kit" came with good quality components (Viair 380C compressor and chrome plated horns) but lacked enough plumbing and wiring to install the kit in my truck. The horn button was also missing.



Anyway, I decided to mount the two horns on the front of the truck as opposed to anywhere else underneath since I wanted the sound to be... well, up front. I chose a location between the two front body mounts, and mounted the horns on a piece of flat stock (after drilling all the holes I cleaned, primed and painted the piece to slow down the rust).







I mounted the horns both facing the same direction to minimize the air tubing runs; in hindsight, I should have mounted them in opposing directions to keep the flat stock from torquing when I bolted everything down. Doing my research I had also read that the tubing to the horns should all be the same length so the horns operate at the same time, but that was for train horns (chimes). Since these horns sound the same a split second lag in air to one wouldn't have mattered.

At first I planned on mounting the tank under the truck, but then I realized the line from the compressor is so short I'd either have to plumb it or mount the compressor under there too, so I decided to mount them both in the bed, Since the tank came with 4 ports on one side of the tank and no drain port (great design!) I wanted to mount the tank in a way that I could add a drain valve. Plus, I didn't want to carve up my BedRug so I could mount them on the floor of the truck. I had some 1" plywood in the garage, so I bought some slotted angle, trimmed the bottom and bent legs into it, them measured the length of the tank, added the compressor, and measured up so that I would hit the ribs in the bed. I mounted the relay on the board as well so the wiring (except for the solenoid) would be in one place, out of the weather. The four ports are air in, pressure switch, 3-way adapter with 1/4" plug, 25' air hose and moisture drain, and the last port is air out.



I routed the relay power, pressure switch power, and 3/8" OD air tubing along the outside of the frame rail on the passenger side (to avoid any heat from the exhaust). I mounted the control solenoid on the side of my Volant CAI; this part came with absolutely no way to secure or mount it, so I used two clamps on the tubing to hold it in place.



Power and momentary switch wiring are routed along the top of the firewall.

After the air leaves the solenoid there's a splitter down to two 1/2" lines for the horn; this stuff just sort of fell into place.



The kit supplier recommends connecting directly to the battery, but I didn't want the compressor running or the horn button being active when the truck was off so I tapped into the 20A trailer wiring on the fuse block.

Finally, I located the momentary switch on the dashboard in a spot where I can reach it easily but where it won't be hit accidentally.



After firing things up I discovered that 3 of the 4 tank fittings supplied with the kit are leaking slightly (even with Teflon tape on the threads) so I'll be looking for some type of pipe dope to try to seal them up.

My advice to anyone who wants to do a horn installation - skip the "kits" and buy the parts yourself. The tubing and fittings can be found at your local hardware store, and you can order the tank, horns, and compressor on line and probably still save over the cost of a kit.