that reducer is just a 4-ohm resistor. to drop the voltage from 12v to 6v the load must draw 1.5A. I have no idea what your horn will draw, but if it isn't in that area the 'reducer' will drop a different voltage and your horn may not work. By the way, in operation with 1.5A, the resistor will dissipate 9 watts and will get quite warm - consider mounting it to a heat sink, like the firewall or inner fender.
