Why not just call your BMW Dealer's part department and ask them to look it up?

That's what I do when I have a difficult time finding particular items. The parts guys use the system everyday and typically know where to look.

Have you confirmed it is a separate wire with connectors on both ends, rather than part of the main wiring harness? If it's part of the main harness, you might have to "build your own" using their individual connectors and a splice.