I believe that the parking lights are on with the headlights. I'd have to check the switch I have in the garage to be sure. But pulling power from the parking light terminal on the fender would work if you run from there back to the switch, then back to your fog light. You could save a little wire by pulling power from the headlight switch park light terminal instead, then to your switch, which I assume you want in the cab, then out to your for light. Or you could use any other hot wire terminal (such as on the ignition switch side of the ammeter), but the headlight switch would be easier to get to. Just remove the headlight switch from the dash and lower it below the dash to access the terminals.
