[onelove]

BMW is likely supplied the connectors by an outside source such as Tyco or one of the other electronics manufactures who assembled them, it may be possible that polarities are reversed from car to car depending on how the diagram, plug location, etc is designed for that specific vehicle.

Most manufactures in the past (up until a few years ago) simply use a voltage based sensor that tells a separate airbag computer "look there is someone sitting in the seat", which assuming the Z4's is similar will do the same. If it were reversed you'd likely know right away by a seat belt warning light when no one was sitting in the seat, well before the air bags ever got deployed.