when i changed the interior of mine the centre belt kept locking so you couldn't pull it out more than a couple of inches
had to strip the arm rest and seat cover off to be able to find the reel to free it off. horrible job, scraped knuckles etc
easiest way to deal with it is to find another seat back and swap it over.
the belts all bolt to the floor under the seat base. if the belt is missing. it has probably been cut, so you'll need to replace the reel and that means stripping the seat back, best done out of the car. so as said be better to replace it. the mounting under the seat should give you an idea as to where the belt is.
seat back is held in by a hexagonal bolt on the outside edge, two torx bolts to the hinge mechanism in the middle and finally the belt anchor bolt (which will be barsteward tight). you'll also need to pull out the seat base first. pull the two plastic toggles at the front of the seat cushion and then lift the cushion up and forward.
hope this is of some use,
al