I'll give you my install procedure I did.
For my horn I needed 1/2" ID airlines - on my long bed there was just enough room to mount a 9 gal tank between the cab and wheel well - drilled and tapped holes into truck frame to bolt it to the frame. Mounted twin viar compressors in the truck bed - drilled holes through bed and put rubber grommets in them for airlines to pass through to the tank. Fused and wired compressors off the batteries. Also put a switch in the cab for power feed to compressors - compressors were triggered on and off with pressure switch, but they do go bad, so ability to cut power manually and or feed the tanks manually is a good option, as well as a mechanical pressure blow off valve on the tank for any over pressure issues. Ran airline from tank to a quick connect at the back of the truck for auxillary air. Ran air line down the frame rail (350 psi airlines rated for outdoor use) to a 90 degree bulk head fitting. Removed front passenger door A arm plastic and drilled a hole through the floor for bulk head fitting so all is weather tight. Then ran air line under dash and made a bracket for a metered train horn valve under steering wheel, the ln repeat bulkhead fitting process behind driver door A arm plastic and air lines back to the Horn. I was able to bend some flat stock to make brackets and bolt the horn to the frame between the fuel filler tubes and the rear wheel well, pointing the horns down to the ground to get good sound bouncing off pavement. Some guys remove the spare tire and put horns there, but didn't want to give up the spare.
Couple weekends of work and a PIA - but that said it was an awesome set up and I miss having them in the truck immensely - there is no better look on someone's face then when they beep there horn and flip you off, then to let the horns blow and wave with a smile
My horns required 1/2" ID lines to have enough CFM and a minimum 8 gal tank and constant minimum 150 PSI to sound off correctly.....some don't require as much.
The metered train horn valve is also "tits" as you can feather in and out and make it sound really good as opposed to just a horn blast.
Hope that helps
