I had this on my wife's 206cc. The reversing light switch on the top of the gearbox was faulty. A common fault. I bet your switch is the same or similar (same PSA group cars).
You can check the switch with a multimeter if you like.
I bought a new switch from the main dealer (about £7) and fitted it myself. No problems since.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.