The same ceramic resistor that's used for a 12V ignition coil can be wired in series with a 6V horn running on 12V- - - -problem solved.
Jerry
I tried the ballast resister on my dual 6 volt horns, and they would not work. I hooked them up to a full 12 volts and so far this last summer, I’ve only used mine to show people how nice (and loud) they sound!
I bought an original looking relay
here. 
