Seems I have the only Stebel Magnum that actually works.
When I got mine about 2 years ago, everybody and his dog sold them. Now they are like rocking horse droppings, and the online reviews say they are very poor quality and usually don't work out of the packet.
Get something made by Fiamm perhaps, the motorbike boys are always after very loud horns that don't take much current or space, try a few 'bike accessory places.
On previous cars including my last car which was a civic, i used two old nissan horns from something like an old sunny, got them secondhand, a quick tap with a small hammer to knock out the rust and wired up in parallel, always sounded great. When the weather, sorry IF the weather, improves I'll be hunting some down from a breakers to use on the note. I'll try and upload the sound before and after somehow.
as i said above the piaa horns are loud, with the pre facelift you have to lossen the bumper off to get to the horn on the face lift you can unclip the grill at the top get you hand in and release the bottom of the grille http://www.youtube.com/watch?v=_Ndc9fN3twk400/500 hz piaa horn