I don't have eLearn for the 159, but the Brera eLearn might be similar?
On the Brera, there is a small loom which runs across the bumper for the parking sensors and rear number plate lights. This loom terminates in a round connector called D212 on eLearn. Pins 11,12, and 13 of this go to the lights.
From this connector, pins 11,12 are connected to the Body Computer (Rear) at Pin 9 and 38 respectfully, and pin 13 goes to the Right Rear Earth point. So for a single point of failure, I would look at the earthing point, the connector D212 (especially if the parking sensors are also problematic) and finally the Body Computer (Rear).
The D212 connector appears to be located on the right rear side of the car, near to the bumper.
The C031 Right Rear Earth point appears to be located nearby to D212
The Body Computer is located beneath the fuse box at the steering wheel.