This is one of those things that has magically healed itself in my car, 1991 300CE. The drivers side extender would extend but not retract. You can just gently but firmly push it back in. I would then just remember to buckle up before starting the car. Of course I would sometimes forget and have to push it back in. Then sure enough after a week or two, I'd forget to buckle up first and it would extend but, Viola!, clip in the seat belt and it retracts! Been fine for the past year.

Similarily, my fuel tank needle was jumping erratically around the 1/2 tank mark. My habit was to not fill the tank completely, usually empty to 1/2 full. I then started to keep the tank between 3/8 to full and no more jumpy needle.

glenmore
1991 300CE