C4: Door open/close sensor not working- how to repair?

[c4tobe]

I have a C4, 2004. One of the passenger doors at the back seat has a problem. It opens and closes normally with central locking system, but the sensor of this door is not working. When I open the door, it is supposed to light up the cabin light, and a message appears on the screen saying that this door is open, (like it does for other 3 doors), but it does not!

Anyone knows how to repair this? I have been told that the sensor is within the door lock mechanism, and I should the whole mechanism. Any cheaper solution?

[Hell Razor5543]

I would spray some thin oil (NOT WD40) into the lock mechanism, to see if that loosened things up.