The reverse lights on my CE 300 do not work, bulbs are fine, if you remove bulbs there is 12v on the cluster when you select reverse but as soon as you put bulb back in voltage dissappears and lights do not work, the earth seems fine as all over lights work. I am wondering if there is a problem with the lamp failure box which is online with reverse lights, but I cannot find this circuit box, can any one tell me where this is located

Reverse lights do not work, car elictrician says it is gearbox switch , but is specialist job, can anyone tell me whats required to change this switch, Im no mechanic but I am pretty good at electrical stuff