Rear center seat shoulder belt repair?

I recently purchased a nice clean 2007. However, where the center shoulder belt runs down inside the top of the seat back, the plastic piece around the this rectangular opening is missing. The belt is just passing down into the back of the seat. I have a set of rear seats in a parts car but they are of a different color, however, I might be able to salvage this plastic part if I knew what is involved in disassembling that much of it.

Has anyone ever removed this part or opened up the rear cushion for this or other reason. Any tips would be appreciated.

Steve