Installing aftermarket horns is a fairly straightforward operation.

Fiamms are the overwhelmingly popular favorite. Either in tandem with the OEM horns through an added relay, or as a direct replacement.

Whether you opt to replace the stock horns, or add to them, you should use a fused relay for best results. A 42k jpg of a wiring diagram courtesy of Guy Young, and a 4k gif diagram from Bryan Moody.

This is what the final install looks like:

From Guy Young, replacement of the OEM's on a post 94 model. 24k jpg

From Mark Cipriano, additions to the stock horns on a pre 94 model. Port and starboard pictures, 40k jpg each. Note the brackets that mount to the dash bracket.