I purchased the 66 21 0 303 083 ADDITIONAL KIT FOR CAMERA last year from my local BMW dealer. At that time it included the necessary wire harness, and I would be very surprised if it doesn't today.
I'm very happy w/ the quality of the OEM camera. Mine is a model or two old now (purchased in 2005), so perhaps they had better quality ones back then?
The video switchbox included in the OEM kit has 2 video inputs that I've verified. Likely it has 4 total. The input for the rear camera has 2 triggers, one for when you select Reverse, the other can be actuated at any time and automatically turns off after 30 seconds.

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2010 X5 3.5d: SOLD, but miss the torque...
2004 X5 4.4i: SOLD, but served us well