you can run 12v on a 6v horn no problem. It might burn it up if you were to lay on the horn for a few minutes straight but, who does that? I ran 12v through my 6v horns in my DeSoto for years. Run 12v, not 6 as a test. You may also need to use percussive maintenance on it while applying 12v (give it a whack with rubber mallet or something)if something is stuck inside, that should jar it loose. Yo may also grind the rivets off and clean it up inside (I think there may be a pair of contacts inside that can corrode). Yo can replace the rivets with small (looks like 10-32 bolts) or use pop rivets to put it back together. That extra little box on it can probably just go away, it may be a relay but,
