Rear light cluster, 2007, 10 series, three red lights. Outer most and middle tail lights, outer most brake light. What is the third red light nearest the reverse light used for/by, as the right hand reverse light is being used a rear fog. Can’t figure out whether or not it is supposed to be rear fog, or has a use on a later/different model but using same lamp cluster.

Winter pack fog light on the 10 series. 

The wiring socket should be there (the existing sidelight on the 'middle' lights has an additional unused wire - that is the fog live) 

Thanks for that Chris.