There is a sensor on the side that you need to check is aligned (for the bottom half that pops out). if this sensor is not seeing the clamshell closed, then it will not allow the rear to close.

Now, if you can't open it either and the button on the center console area doesn't work either... and you've checked in I-Drive that its not switched off (as there is a high limiting feature) then you probably need to visit the dealer as its something more complicated.

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SOLD - 08 X5 (Titanium Silver w/Black Nevada with all options).