A 12-vdc automotive horn draws so much current you'll probably need to also use a 'horn relay' wired into the circuit appropriately. Just be certain the electrical current capacity is sufficient without going too far overboard---you don't want a relay sized too low ampacity but also don't want one so physically large you can't find a good place to install it.
A relay, the horn (tested before install), some 12-ga automotive wire, and a momentary-type normally open pushbutton switch should do you. An auto parts house should have it all, perhaps in kit form. Be sure in any case to fuse BOTH sides (the relay control circuit half and the horn power half) of the circuit--cheap insurance.
A low current draw (perhaps no relay required, depending on particular buzzer type) option would be a piezo-type 12-vdc buzzer. Some are extremely loud and penetrating, even moreso than a traditional horn. Look at
www.AllElectronics.com website for inexpensive pull-offs.
If you really want to spiff it up go with an electronic 200-watt noisemaker similar to a modern siren. Refer to
www.Galls.Com.
Please post back your experiences so we may all learn. Sounds like something that might be popular with other owners.
