rear lights not working! ford mondeo 2l (1999 V)?

can anyone help?

the rear lights on the driver side don't work, the indicators, fog etc all work it is just the two bottom lights!

i have changed the bulbs, no luck and i have tried another light box (the bit that you plug the lights into) still no luck! What could the problem be? where should i look?
any help would be great! many thanks.