How do you change a drivers side seat belt on an 89 MR2?

Hold the assembly firmly, in a vice if possible careful not to crush it, and SLOWLY pull the belt from the retractor. It should come out. If you jerk it it won't , that's the way it's supposed to work. It does not matter if it's bolted to the car, either it will come out or not

Thanks DavidH25. That's one thing I haven't tried. It looks likes it's in good working order but like I said, the belt won't retract or extend. I do this over the weekend and let you know how it works out.

Please do. I'd like to know

From what I've heard the mechanism has a magnetic latch inside that prevents it from extending unless it is up against metal (such as the metal part of the car where it is bolted to). So try that perhaps.