The old style hatch lock and handle is a weak point for these models.
Possibly you could use some penetrating oil to make the lock cylinder work again. At that age it's not unlikely the handle itself is beyond repair.
If you plan to keep the car, the rear handle could be replaced for the newer, wider all plastic looking thingy.
It's not bolt on, you need to drill new holes in the tailgate, but I managed to make the old lock cylinder fit in a new handle.
It's a much better design that doesn't let so much water in the lock cylinder.

HTH
mila