Thanks for the replies. I have solved the isolation of the first horn making nylon mounting parts that insulate the horn from the mount itself.
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. If I had a 6 volt battery I could measure all of that as I have a watt meter that handles up to 60 volts and 100 amps DC. Its display shows volts, amps and watts and the unit is about the size of my hand.
