This is what you need.
It's the only system I've seen that integrates an airhorn with the OEM horn instead of replacing it.
Dunno how this got to be a horn thread, but... that Banshee looks to be the standard compact Nautilus horn. I've had lots of them and have a love/hate relationship. I love their loudness. I don't love the pause before they start up, although it's very short. But I really hate the fact that they don't tolerate water at all. I even spent big bucks on their nautical version that was supposed to be water resistant and it lasted about half a winter in the rains. I finally gave up after about my fourth horn.
After reading tons of reviews, I went into my local Kragen's auto store and found a twin-tone electric setup that's plenty loud, instant on, and has held up in the rain, even mounted low on the front fork. And they weren't expensive at $20. A quick tap is fairly polite and a prolonged button-press can be heard inside the SUV's on the freeway as they obliviously drift over the braille bumps between lanes while chatting away on their cell phones. Bless their little hearts!
Something like these:http://www.amazon.com/FIAMM-74100-El-Grande-Horns/dp/B000DLBIGG
And Flar, I hear your pain. The BMW Teutonic switchgear logic almost drove me crazy (short trip!). I've heard their newer bikes have finally overthrown the German infallible wisdom in favor of the heretical Japanese norm. Maybe an engineer finally died.