CE 300 24v 1992 Reverse light failure

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