This happened on mine---actually it was exactly as you describe when I bought the car. It turned out to be a lot bigger job to fix it than I expected. First I had to figure out how to get the latch to release in order to remove the back rest (it was the one one on the left) It turns out you have to pull one of the control rods and push the other at the same time. Once it can be lowered, you have to depress a spring loaded pivot pin at the on the outboard side, and perhaps unbolt the hinge assembly at the center. With the backrest out of the car, you have to skin it, and dismantle it completely to install the replacement parts. If you can find a matching backrest with a working mechanism at the junkyard, that makes the job go a lot quicker. Then you can just swap it.

Note to all 745 owners----the backrest release mechanism is fragile, with several small brittle plastic parts involved. Be gentle when operating it. You don't want to have to do this job.