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I installed one onto my mitsubishi lancer a few months ago. The hardest part is finding room to mount them. I removed my alarm siren just to mount it . I also made a small bracket and drilled it into the firewall with a self tapping screw. Works fine.
Not sure about how you'll mount it into the holden engine bay as there isn't a huge deal of room to add trumpets and it'll be a real pain in the ass if yours is a 5 trumpet horn like mine :wax: Good luck with it :thumbsup:
As for the actual install, once all the horns and compressor are mounted you just get some 8mm hose (may have come with package) and soften the ends with hot water so it's easier to slip over the hole. So that should set all the actual horns up but then to wire it up, you need a separate switch from your car horn as it's illegal to replace your standard car horn with a air horn, and mount that somewhere in the dash. I just used a small round push switch. Pull your wiring through the firewall (harder than you'd think. I used a long metal wire to get 50/50 through then tied the wire onto the end of that and pulled through) and hook up your switch. I soldiered mine on so if your gonna do that, do it before you fully mount your switch.
Hook up your relay somewhere in the engine bay and put the other end of wire you ran through onto the relay. From there you can simply just run your negative wire onto a part of the chassis and you power through to the battery. Make sure your power wire doesn't touch the chassis when juiced up so try to keep it neat and tidy. You'll want to run a small fuse on your power wire too. You can get this and the switch from Jaycar for about $5-10 total depending on what switch you use.
Test it out and should all work. Good luck with it. If a 16 year old can do it, I'm sure you can.
As far as I was aware, you were not allowed to replace your current horn with an air horn. The air horn had to be fitted to a separate switch from your original so that your regular horn still works. That's why you can't use original wires.