Wiring additional car horn, how can i switch between factory and additional?

Im installing a monster air horn in the rear of my rubbish car. I can wire it on its own working independently not a problem, however what i want to do is activate it using the existing horn press switch, but fit a switch to the dash so i can select between the factory horn and the air horn. Can anyone help with... show more Im installing a monster air horn in the rear of my rubbish car.
I can wire it on its own working independently not a problem, however what i want to do is activate it using the existing horn press switch, but fit a switch to the dash so i can select between the factory horn and the air horn.
Can anyone help with some diagrams or info please?
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