ItsStock02:
You can remove the Torx-45 bolt that holds the seatbelt guide to the side pillar. That will give you some slack to see if the belt can be freed up. If that doesn't work (i.e.--belt remains locked) then you have to take the back seat out, the side panel off, and remove the retractor assembly and replace it. I say replace it rather than repair it because it is a safety device and you don't want to use a faulty seatbelt retractor! (Well, your passenger doesn't.) For information, the white plastic bobbin that the belt winds onto and off of breaks and then the pieces get lodged in the retractor mechanism or the belt binds on the metal wall and slows it from properly retracting. Replacement is the only solution I have found to work.
HTH,
Chris