You might be interested in how I added another horn to my car. This
is the switch I used. The fog light button housing is hollow, you can drill a hole and add a switch like I did. There's a screw underneith the vent, the whole vent asembly pops out, then you can remove the switch connector and work on it. I drove around for about a week with the switch missing, the car didn't know
I ran one part of the switch to the car's ground, any bare metal part of the chassis. I ran the other part through the firewall under the deadpedal where a BUNCH of wires goes through, through the fusebox, snaked around my engine bay to the other side, and to a relay which was also wired to the positive battery terminal post. Relay was hooked to the horn.
I still havn't figured out where the normal steering wheel horn wire is and if it's positive or negative, if it's negitive, I can just tap right into it and my normal horn siwtch on my steering wheel will activate both of my horns, and the alternate switch can give me some really quick "beep beep" horn control of my louder one
You NEED a relay using this switch method, horns can draw a lot of current, upwards of 10 amps depending on what you have.
If you find out, Please let me know because I'd love to have a "GET THE F@&# OUT OF MY WAY" horn at thumbs reach
BTW: Ali G is the shit.