If the central locking button does not work, can you unlock the rear doors using the key in the drivers door lock cylinder?
The only reason I can think someone would have disconnected the door lock actuators would be because of auto locking enabled, this would cause the rear doors to lock??
But I would check all the fuses carefully to make sure none are blown and none are missing or in wrong position.
You could also pull the GM module and make sure all connectors are plugged in all the way, I do not recall if the door locks are separate connectors from the others. The
www.bmwgm5.com site may have a connector diagram.
But either way seems like if you need to go after door lock actuators, you will need to pull the door panels and you will find out if they actuators are unplugged for some reason.
Good luck.