I had not though of putting a voltage reducer before the relay. I am guessing I would need a reducer that would handle at least 20 amps but I don't know how much current these horns draw.
That would be the challenge. A reducer that could handle the high amperage of a horn circuit would be large with an equally generous aluminum heat sink. One would want it in close proximity to the relay (to minimize resistance from wire length) but out of site from an aesthetics point of view . . .
