Check to see if you get switched 12vdc back there when you push the button on the dash. If not, check to see if you have switched 12vdc on the output of your DECC box under the dash. It would be one of the orange wires, mine have screw terminals but others are hard wired, you'd have to check it on the plug. If you have 12vdc, then measure the resistance to ground at the bulb socket. You either have no 12vdc or no ground.
