Rear cam

Hi guys I went to look at a 55 plate supercharged vogue se today. Seems like a good one. I've been looking for a while and looked at quite a few so now have an idea what to look for. Everything seem to work ok, checked air suspension in all hight levels quite a few times and seemed to work quickly and efficiently like it should. Checked heated seats, aircon, heated steering wheel, electric column ect. Looked in the boot electronics for damp which was dry and nice and clean ect. The only fault I could find was the reversing cam didn't seem to work. When put Into reverse the screen changes but just to a black screen. Had a look at the cam and didn't look damp of misted up. Not sure weather the crawler cam was working aswell but that might have been me not doing something right. Is this common with the rear cam? Anybody know how to repair? No jokes about learning to reverse with the mirrors please lol I've never had a rear cam before and don't need one but it would be nice to get it working as it has it. Apart from that car seemed like a good one for its age and not too much wear inside. Any thoughts guys?

Thanks andy