I use a 8 gal. portable air tank that is mounted in my cross frame tool box. You could mount the same in a pickup bed and hide it in a cross bed box. Run the air line under the cab (you will need to drill a hole in the bed and tool box) to wherever you have the horn mounted. I have mine in the engine compartment. It is a single 26 inch long Hadley with a 5 inch bell.
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1996 3500 Clubcab SLT, auto, 4. 10 limited-slip, TST #5, 370s, AFC spring kit, Psychotty, Dunrite converter, Trans-Go, DiPricol EGT, boost & trans temp. gauges, 9 ft. aluminum flatbed and tool boxes with Venco dump. Custom front bumper, Hadley air horn, PowerVision mirrors & 4" Stack Exhaust.