Mate I had the same problem..The rear lights on the rs turbo are a disaster the best of time...water get in at the top of the lamps and as turbo-jon says and rots the connectors....try rubbing down the connectors with sand paper and re-connect and try...if this doesnt help try your earth....if they still dont work then buy new bulb holders from ford..their dirt cheap so say the bullshit hassle and buy new ones....