I don't know the specifics of your car, but I mounted four extra horns in my W126. I drilled holes into the blank sheetmetal in an unused space in my engine compartment, and mounted them using the brackets that came with 'em. As long as the horn is somewhat close the the front of your engine compartment, and aimed towards the grill, it should blast pretty well.
The real trick will be wiring your new horn, expecially if you want to wire so it'll work with with the stock horn (i.e. push horn thing on steering wheel, both horns sound). I'm not sure how that'd work. Wouldn't be as hard though if you wired it to a separate switch under the dash or elswhere (that's what I did).
Hope this helps.