Q: door is not unlocking with the central locking and the lock seems very stiff when trying to unlock manualy. Is ther a way to open the door without causing any damage please ?
A: Your electric lock motor pushes the lock up and down. It has become immoveable, and should be replaced. The central locking works on the other 3 doors and the boot? right? While you are still able to manually lift the lock and open the door, remove the door fabric panel and look at the electric "lifter". It doesn't work. You could make extra work for yourself and remove the panel on a "good" door, and see how it is supposed to happen. On a 306, the front door motors have 4 wires each. The two rear doors and the boot have only 2 wires for their electric locks. I think they are all "grounded" to the car body. I think?? I would say, JUST YANK IT UPWARDS and break it if you must, it needs replacing anyway. You have to open the door to replace it.