I bough the mini Stebel airhorn. I can tell you it doesnt fit anywhere easily. I am sending it back. Instead I just ordered the wolo.
http://www.wolo-mfg.com/elec.htm Scroll down until you see the chrome Wolo Maxi MODEL 325-2T and you can hear the sound. They look like they should fit where our stock one fits.
I plan to buy a 30AMP relay and a 15AMP inline fuse (can get both from Radio Shack).