the empty socket on the passenger side corresponds the rear fog light on the driver's side which is illuminated via turning the headlight switch fully clockwise then pulling it toward you. Do you have an owner's manual? Apparently a rear fog light on one side is though to be enough by the Eurpoeans (in the US we don't think any are necessary?). Likely MB's sold in Great Britain will have the ligh on the right and the empty socket on the left.
regards,
Mark