How do I add or change over the rear fog warning light
 

I have an English W211 saloon in Spain. I need to add or switch over the rear fog warning light so I can register the car on the Spanish system . How can this be done you clever people .
On my clk I ran another wire but as the bulbs are all in place I suspect it is a software adjustment.

Could any one advise me please?

I am getting a little desperate, the local Mercedes Garage want to rebuild my car