Ahh that must be that middle button in between the headlight controls on matrix cars. Saying that I've only properly used my fog lights in previous cars (my Volvo didn't have them, but it was never foggy enough) maybe twice where I couldn't even see a car 3-4 carlengths in front on a 40mph road and had to use the Sat Nav to give me a heads up on which way the road would be turning. Why are you looking to retrofit them? Does it get plenty foggy where you are at?