There is a metal pin that sticks out, similar to the door lock i suppose. You can probably see a hook on the rear seats where it's supposed to latch on.
I'm not sure how the pin is attached to the car though, i would of thought it would be fixed to the bodywork and not the plastic trim, afterall we all know how brittle this old French plastic can get somehow i don't think it's strong enough.
But i could be wrong.