Unsure about the convertibles, but in the hatchback there is an extra metal brace part that is welded into the hatch. If you were to add 3-pt belts to a pre-90 hatchback, you'd need to drill out the spot welds, remove the bracket, and then weld it into the body of the hatchback. That gives you the upper mount for the 3-pt belts. You also need to replace your interior trim with that from a 90+. This might mean some dying of the panels to match colors.
Now, that's the hatchback, so unsure if the convertible is the same but i'd imagine it so.
The 91+ beets had the upgraded belts. You’ll need all the seat belts and mounting stuff from one. They bolt in but the mounting point is not there on yours. You’ll have to either weld or bolt in reinforcements.
I am told the seats are different on the convertibles in 1990 and above as well, and that the salt belts come through a hole in the seat and that the seats are n't as easy to mount
Yes - the part/bracket/component for the upper part of the rear seat belt ...and if the other anchors are also the same when converting from lap to lap sash