In the E46 my USB to IBUS interface has the out (software) for the ground. I believe its built into the interface.
IBUS Interface for BMW cars
On the GNET module and CIS-IBUS its also built into the interface. If you are making your own you will need to probably use a relay as you stated. There was a wire from the GNET interface out the backup light.
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E83 - Aero PKG Mods installed: Homelink mirror. CIS-IBUS video module, AUX, Rr Dr Release, stock wheels, Remote start, CarPC (Atom) & mud flaps - NAV - MKIV - NA Map DVD - 2011
E46 - 18.5 wheels/tires (summer), 17 stock (winter), Homelink, NAV software upgrade, BMW video module, Radio to V53, AUX, USB IBUS interface,BMW alarm install, CarPC(Epia), remote starter, tank battery & angel eyes. NAV - MKIII - NA Map CD - High Version 2008.1
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