I'm the guy that did the Caddy horns, if not the only guy. I love my set up, loud without being obnoxious. They helped save my car from being totaled in the school parking lot last month, because no one can see a bright yellow car.
Here's one way to do it, if you want to make it a secondary set up.
(You can get power straight from the battery instead of the starter solenoid on a Focus. Also the diagram shows a one-wire horn that self grounds to the frame, if you get a two wire horn then just ground the ground wire.)
The sound of the Hellas don't do it for me one bit, they just look good in the grills of Subies.