A regular vibrator horn will work OK for short periods of use. I've never tried a 6v klaxon on 12v. If it were me I wouldn't do it. It might be OK but you might fry it too. Maybe someone who's actually done it will chime in.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.