1. Your horn should work with 12vdc one side and ground on the other : the horn is not polarity sensitive.
2. With key in on position test to see if the tan wire is ground or the 12vdc. Bike might not be wired with OEM color code.(colors I'm used to are Yellow/black and black)
3. Best test will be to disconnect wire from horn and then ensure you have a confirmed ground on one side and 12vdc on the other; horn should turn once wires are connected. If it does not work replace horn>>> just my 2cents.........Good luck