On one truck I had a Grover compressor, which is what's used on many fire trucks. It's the real deal, very nice stuff that's American made and pretty pricey. Its crankcase is oil filled and it lasts. I found it on ebay.
I had it piped to a wonderful and haunting Nathan K5H horn a locomotive. You can imagine where that came from. Nice setup with a manual valve that could throttle the air.
There's a lot of Chinese garbage out there. You get what you pay for. Either pay a small fortune for real stuff or find it very used for still a high price or buy cheap crap.
On a first generation Expedition, the pump was under the hood and the trumpets were separated behind the bumper. The tank was on the frame rail.
Really a beautiful note. You could really hear it for miles.