Let me start out by saying that I kept the stock horn and it still uses the steering wheel switch. It is used for friendly "hey the light is green and you're still sitting here" beeps, and for emergencies for which the horn is intended. BUT,
I did install a train horn with a seperate button on the dash. This is used to scare my friends and have fun with... I don't use it on the road. I think it's illegal anyway...
I put a 2 gallon tank and small air compressor behind the interior spare tire rack. I have 4 horns pointed straight down (so they aren't directional, just as loud from all directions) between my rear bumper and my rear axle. I couldn't find room under the hood plus it was less airlines to run/deal with. Longest horn is 20" and shortest is 13", all have 5" flares at the end. The horns empty my 2 gallon tank with 140 psi in about 4 seconds... That sounds pretty quick but cam seem like an eternity while your filling up your shorts.
If you want that cool train sound without shaking the ground, you need at least 3 trumpets, even if you don't go as extreme as mine.
Plus, I can air up all my tires anwhere, anytime.
Here is the kit with the horns I have, but I have a different tank and pump.
http://www.hornblasters.com/products...train-horn-kit