I can help a bit.......
You can use the frame for a ground connection, the negative terminal of the battery is still tied to the frame even with CANBus.
The brown wire on the horn is ground ... for whatever reason BMW chose brown for ground connections. The other wire which is green-yellow I think is the "hot" lead. The CANBus monitors current draw for most devices (not 100% sure it monitors the horn), but you might want to leave the stock horn attached anyway for redundancy and tap the green-yellow wire for the trigger (which "should not" not change current profile enough to piss off the CANBus).
There is a thread somewhere on the Naut. install maybe F800Riders?
JRWooden screwed with this post 07-17-2012 at 04:03 AM