Volvo V70 HU-850 radio no light at buttons

[budG60]

Hi,
Does anyone know what the problem could be when the radio buttons of the radio (HU-850) of a Volvo V70 do not light anymore? Everything works fine and the display light works, but the buttons stay dark. Does anybody have any experience with this?
Any suggestions?

[synkrotronic]

I think is a radio problem.need to remove it,open and see this specific circuit