Solution----in my case---The reversing light on my '09 159 auto TI sedan failed.
There were no volts at the connector to the light cluster.
I guessed that the switch was ok, because when the car was in reverse, the mirrors still dipped and the reversing sensors still came on. So, I checked the boot connector (D040) for voltage.
This is located inside the LHS of the boot under the LHS of the parcel shelf. I only needed to unclip a small portion of the lining under the LHS of the shelf.
The ~10 connection connector is clipped Horizontally. There were volts on the grey/red light duty cable, which supplys the reversing cluster.
Upon checking continuity of the grey/red light duty cable through to the cluster, it was open circuit.
So , I pulled on the grey/red cable and it came out of the loom...it had completely snapped. I used the length of the cable to measure back up the loom to the car side of the flexible/waterproof coupling. Open the loom---horrible sticky felt -- to discover 6 cables with bear wire!.
The insulation had gone hard and the cable was now the flex point! fatiguing with each boot opening....
When I lined up the reversing wire it was about 15 mm too sort.. therefore it was taking the loom strain and was the first to go..
ok, now the best repair is to replace the hardened wires,,, or replace the loom, or....... as I did.... insert 50mm or wire into the reversing wire break. tape up all of the bear wires.
Which holds them straight.
Check out the actual lie of the loom and heavily tape it all so there was no longer a kink. It'll break again--- but its going to be a fair while.

arty: