I recently fitted a Stebel Magnum to my 09 GS with ESA. It is located under the front panel on the left hand side of the bike and is attached via a home made 'S' shaped bracket which fits under the front brake hose retaining bracket. I hope the pictures below help to explain this.
I connected the Magnum horn to the original horn wiring with blue connectors (I aim to change these in the future when I have figured a better method!) and as an experiment
I also reconnected the original horn and was surprised not only in the fact that both horns worked, but the noise they created when used together 
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I am fortunate that a new GS was delivered to me yesterday, but I now know this mod also works on the latest bikes.
Mike
I connected the Magnum horn to the original horn wiring with blue connectors (I aim to change these in the future when I have figured a better method!) and as an experiment
I also reconnected the original horn and was surprised not only in the fact that both horns worked, but the noise they created when used together .I am fortunate that a new GS was delivered to me yesterday, but I now know this mod also works on the latest bikes.
Mike
