Attached to your gearbox is a plug sensor, this operates as a switch when reverse is selected, unclip the cable connection plug to this & short out the connection by use of wire from one contact to the other......turn on ignition you have made the switch by shorting out the sensor if your reverse lights come on you have proved it is the faulty plug sensor, replace new sensor after unscrewing old - refit cable connection to new.
David Williams
September 2008