I would like to fit reversing sensors to my 2006 Zafira Active and have made up a test lamp. After undoing the clip holding the wiring to the lamp holder I have 5 wires.I think the larger brown one is the earth.When I probe the other 4 wires my test bulb doesn't light up although the bulb definately works and the other reversing light is on.I also get an on screen message saying that a bulb has blown.I have tried different combinations including earthing onto the bodywork but still get no reaction. Can anyone help please.