Precision M6500 Keyboard Backlight Stuck ON

Hi Folks,

I have recently rebuilt my precision m6500 - fresh install of the OS (windows 7) drivers and apps. Now the keyboard backlight is stuck on. It's not at full brightness, but I cannot find anyway to turn it off, or indeed change the settings.

In the BIOS the feature is disabled, and this shows as off in Dell control panel, but the backlight stays on!

the Fn key brings up the menu but no matter what I select, the backlight stays on.

I've swapped the keyboard out for one in my other m6500 and it makes no difference, so it seems that there is something wrong elsewhere - perhaps a control or voltage signal.

Does anyone have any ideas how I can either turn the backlight off, or restore the correct behaviour so I can adjust the settings etc? Or am I hosed and stuck with the permanent backlight?