i have been wanting to do the same thing, but not quite sure how. A friend of mine had helped me do it on my Maxima, but I do not feel comfortable with my knowledge of electrical systems to try it on my own.
From what I remember him doing, he somehow spliced a wire to the parking lights and to the fogs, so that when the parking lights received power, the fogs would then have power, but you still had the capability to turn them off if you wanted. It sounds easy, but when I look in there.........there is no way I am about to start cutting wires....