You can remove the trim at the back of the headrest by lifting straight UP. Don't be afraid, it will come up. It's held by steel clips anchored into a plastic base. You'll need to have a socket wrench to remove the bolt that holds the shoulder strap tri-angle in place. Be sure the seat cover you have is made to allow the side air bag to deploy. (you don't want to violate Federal Safety Regs, or insurance coverage)
Remove the bolt holding the end of the belt to the floor and the belt will feed through the guide at the top of the seat.
Or are you talking about the short piece with the buckle?