Reverse Light not working

Guys I didn't find anything about that, so I decide open a new topic.
I own a td4 2001 and when I bought it 6 month ago, the reverse light sometimes work, sometimes not. Then starts to keep flashing when in use. Then finally stops to work.
I checked the bulbs and they are fine. I change the reverse light switch by the gear box, and still not working. I did the volt test, and there is power in the sensor and there is power in the fuse. However I couldn't find power in the wire by the socket in the back, either inside the socket. I am not sure if I couldn't find a nice earth point or if there is no power getting the bulb.

Anyone have a clue about what may be happen?

thanks

Glauco