how to repair a nissan almera tino reverse camera not working?

Hello All. I have a nissan almera tino SVE ( 2003 ). the back sensor is full of gung, it used to work but now all i get is a black screen with a little message on the bottom. do you know where to get a replacement and how to repair it. try main dealer they need an arm and leg ( very expensive )!!!. Thanks for any help Chris.