Instrument cluster back light

[viz007]

Hi. I removed my instrument cluster today to check which replacement batteries I need to buy for the SI board. After I checked this and fitted it all back in everything is working as before except for the back light on the right hand side of the cluster which comes on when you turn on the headlights ie the back light for the rev counter and temp gauge. The left hand side (speedo and petrol) is working fine. The gauges themselves are working its just the light that is not.

I won't be opening it all again until the replacement batteries arrive but based on my description any ideas what I might have done wrong?

Thanks