How do I fix a bad earth on Citroen C3 rear lights?

I have a 2007 Citroen C3 Exclusive and a beep keeps going off and an "info faulty" warning is displayed folllowed by "rear brake lights", "fog lights", indicators", etc. I have been told that it may be caused by a faulty earthing strip but, does anyone know how to find it and/or fix it? It's driving me and the wife up the wall!