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The mullion is screwed to the door in the middle area- you need to remove the 2 screws and then you can reseat it. It is not unusual for these to break, so if it still does not actuate properly after you do this then just replace it. part#12722813AP
Use a small socket to remove that screw- it's just like a bolt head. There may be screws under the 2 rubber covers as well. In any event that is where it is secured to the door.
I do not know the exact size- that's something that is not detailed to that exactness in the look-up and if I were there I would just try my sockets until I got the right one to fit. It is a metric size if you are having no luck with domestic sizes, so you would need to get yourself a small set to remove it. This all boils down to removing a screw, so you now need to obtain the proper tool to remove it.
No rating after all this back and forth over a screw?