using a 12 volt system with the 6 volt horn

[Charlie Stephens]

I assume you meant you purchased the "resistor". Who did your purchase it from? Did the horn work before? Any pictures? It is such a simple part it should work. The voltage drop doesn't occur until there is current flowing so it should be 12 volts on both sides of the resistor without the current flowing. I assume you are wiring from a 12 source to the horn and then to the horn button as a ground to complete the circuit, is that correct?

Charlie Stephens