A tech at the local dealer suggested these locations for unobstructed sound output and an easy place to mount two horns. Of note, I have 3 horns installed now, retaining the OEM location for a Denali Mini Sound Bomb which adds some more dB but also is useful for avoiding fault codes when you use the trigger wire to fire horns powered thru a fused relay arrangement:

http://www.bmwlt.com/forums/rt-serie...two-horns.html