On my 2010 C30 there's a small dome in the middle of the dash, close to the windscreen, and I believe that is a light sensor. Covering that should have the radio screen change from green to black. Perhaps that has failed? Mine will also light up my dials once it starts t get dark if I've left the light switch in the default '0' position.
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2010 C30 T5 – Now sadly gone but not forgotten 
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