The rear foglights are on both (left and right)cluster of your rear lights. If you look at the right rear cluster of lights, it's on the left lowest corner left of the reverse light. Vice versa, it's on the right lowest corner on the left cluster of lights. From what I am aware of, not all 5s in the US have rear foglights, but they are equipped in Europe and Asia. If your car is equipped with them, you should be able to turn them on on the same knob in which you turn on your foglights for the front, there should be two more modes.
hope this helps...