Get someone to look at the lights, switch on sides, rear fogs and hazards. Then apply the brakes. Look at which side goes bananas, that is the one with the earth problem. Take out the bulb holder and disconnect the wiring plug. Look for white corrosion on the earth tag. Clean all the connections and you should be fine

Expert burns receiver.
Bodger Extraordinaire!!
Old Fords never die, however, mine are on life support
Fallen in love/hate with Mk6s