My 1994 Geo Metro XFI
Ooooooga Ooooooga! :-)
Stock Horn Back Nut Removed New Horn Mounted to existing bracket This is where the bracket attached

It was a lark I decided to do this today. I forget to get a before picture but its pretty clear.
The 3rd pick is the bracket attached to that hole in the 4th picture. Its a 12mm Nut holding it in place.

The Nut on the bake of the horn came off with an 11/16" wrench.
I grabbed the bracket with vicegrips so I could leverage enough force on the nut to loosen it.

The new horn fit perfectly onto this spot. The wires are simple push on crimp connectors and the stock connectors FIT the new horm! (white is positive) I had to find out with a meter since they were both black and after I metered them I noticed I "wiped" enough dirt off to notice one was white.

Works a treat. Listen below! I may get another one to add even more punch and depth.
At only $10 at HF its worth it just to see what it sounds like :-)

New Horn in place. Bracket needed small bend and horn tilted down so nothing jangled or hit the hood etc..