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 ?

Your electric lock motor pushes the lock up & down. It has become immoveable, & should be replaced. The central locking works on the other 3 doors & the boot? right?

While u r still able to manually lift the lock & open the door, remove the door fabric panel & look at the electric ''lifter''. It does not work.
You could make extra work for yourself & remove the panel on a ''good'' door, & see how it is supposed to happen.

On a 306, the front door motors have 4 wires each. The two rear doors & the boot have only 2 wires for their electric locks. l think they r all ''grounded'' to the car body. l think??

l would say, JUST YANK IT UPWARDS & break it if u must, it needs replacing anyway. You have to open the door to replace it.