Dunno about the 123 chassis, but on the 108, you cant take out the backseat without unbolting it from the trunk!

On the 108, the trunk latch mechanism lines up with the lock cylinder in such a way that when locked, the button doesnt push on the latch - only when unlocked. It's such a simple design MB may have kept it in the 123. If so, moving the key while the button is depressed may work. Find a point where you can't turn the key, the nrelease the button, turn the key a little more, and push. Otherwise you may have to have the lock cylinder removed so you can manually push the latch.