Reverse light switch normally situated on top of gearbox. New switch is about £8. Suggest undo connection, remove switch, check for continuity in both positions to see if it is faulty. If it is, buy a new one and fit it.
I have a 1.6 16v petrol 206 (same PSA group), and the job took me about 1/2 hour including removing the airbox to gain better access to the switch.
ChicksFan, March 2010