Get someone to stand at back of car, and check all the rear lights If strange things happen, like the side lights come on when lets say the indicator is switched on, or the indicators light up when the brake is depressed, then this is most likely a bad earth... you need to check the brown earth wires are tight, clean, and not broken, and should be making proper contact with the body
I cant remember exactly where the earth wires are, but they will be in the boot, close to the rear lamps, trace back from the lamps, and you should find them
Hopefully this will sort it
Thom