prob the reverse light switch, quite common on vw's, its situated ontop of the selector module on the gearbox, its easy to find, its the black switch at the very top held on by 2 10mm head bolts, not expensive from vw, try removing the switch and bridging the two wires in the plug, if the reverse lights come on the its the switch, if not then check for power and earth, £10 says its the switch lol