Tailgate pushbutton in the center console control panel
WARNING: This page is about a different car, the 2013 BMW X5, 2012 BMW X5, and 2011 BMW X5. However, it is still accessible from the selected car via links, so may be relevant.
The tailgate pushbutton is resistance-coded. The status of the tailgate pushbutton is of the by the control unit of the air conditioning system. The control unit of the air conditioning system sends the status across the K-CAN.