.......................................................but it did!
Today I installed a Magnum horn to my 09GS. I piggy backed the wires using the blue connectors to the original wires to the BMW horn.
I knew that would work, but this time I did not disconnect the original horn.
Surprisingly the canbus did not cut in
and the low Magnum and high original horn combined make a hell of a noise! 
Hope this helps someone.
Today I installed a Magnum horn to my 09GS. I piggy backed the wires using the blue connectors to the original wires to the BMW horn.
I knew that would work, but this time I did not disconnect the original horn.
Surprisingly the canbus did not cut in
and the low Magnum and high original horn combined make a hell of a noise! 
Hope this helps someone.

