I did it.
I bought an air horn kit from Rickson Truck several years ago. This is the third SUV I've had this system on. On my 07 Expy, I mounted the air compressor in the compartment in the rear on the drivers side where the fuel cutoff is. From there it is plumbed to the reservoir mounted underneath by the spare tire. There is a solinoid valve mounted on the reservoir that is plumbed to a twin trumpet horn that is mounted on the side of the frame rail on the passenger side underneath and between the front and second row seat. It is well concealed and protected there. I mounted a foot switch on the floorboard that I can activate with my left foot.