Another thought, is your horn button wired where one terminal is hooked to a good ground and by pushing the button you are a sending the ground to the relay or the horn itself? If you are not using a relay the horn has power from the ammeter and receives a ground from the horn button whether original or a add on button. If using a relay the ground from your horn button just switches the contact in the relay from open state (NO) to closed which will send power to your horn instead of a ground which means you have to ground the horn then and remove the power from the ammeter to the horn and route to your relay and run a wire from the relay to the horn from the relay output. My horns have a single terminal for power and are internally grounded so the ground is accomplished when mounted. You can also use a simple Bosch type relay which are readily available and cheap.
