The best place is to replace the stock horn. You need to open up the bottom plastic cover under the left front corner, under the bumper. A few screws and bend it out of the way. It will help if you can put the front wheels on ramps for access. You will see the stock horn on a bracket. Unplug the wire from the horn and remove the screw holding the bracket to the frame. Next, remove the stock horn from its bracket. Now mount the Hellas to the bracket so they will fit back under the fender. You will have to make a wire to connect the two Hellas together and to the stock wire. Bolt it up and connect the wire.
Good luck!