Rear center seat belt won't extend

[ApocGold]

The rear center seat belt will not extend. The winder will retract if properly encouraged, but I have to manually move the lock lever to extend the belt. I found a white v-shaped piece of plastic in the seat belt mount box but have no idea where it goes. Is there any way to remove the seat belt winder assembly without removing the whole rear deck? What actually controls that locking mechanism? What can I do to fix it?

Edit: Update-removed rear deck, removed seat belt, found home for little plastic piece (pushes against lock block). Still won't extend. Thinking that it may be the mechanism inside the black box and I don't see how to remove it. May have to get a new belt. Any other suggestions?

 

[doomsayer559]

Had a slight problem yesterday after bottom seat was off. make sure the belt is all the way through the hole in the seat. the plastic at the bottom of the belt should be sticking through. good luck.

 

[ApocGold]

Fixed it. Had to push out four little black pins to remove the black cover. Found that part of the locking mechanism had moved too far, keeping the lock bar in place. Flipped it back and it worked fine.
On a related note, I could not find the center seat belt retractor at any of the places that I went online.