Gauge light trouble

My speedo and gauge lights have been out for some time and I'm trying to fix the problem. Have found some blown bulbs. Some people have suggested a fuse, but all are intact (I cannot identify a separate fuse for the lights anyway) I have removed the switch but am not sure how to reconnect the plug on the back. There are 2 prongs but 4 sockets and a blank. Wiring diagram suggests I should connect the red wire and red/white? And should there be an earth?

My dash lights keep melting..... Think I might try those LED ones off tinternet next.

Make sure you get the correct wattage bulbs. Most places sell 5w bulbs, which are too powerful - they heat up and then melt the holders. You can convert to 12v LEDs if you fancy a little project.