I have a copy of the diagram which I have checked... it shows that the gearbox management computer has a relay that turns on the reversing lights.
If you want to take the signal from the control computer in the passenger compartment, then it's pin 1 on the yellow connector on the underside of the computer (wire is 'marron', think thats brown in french).
Otherwise you can take it from the lights. That's what I did when I installed a parking assist thing. See info here (sorry only in Danish):
http://dinsen.net/espace/teknik/je/parksensorThe kit I used is powered from the reversing lights connection.
- Anders