Turn your truck on, turn your lights on regular beam and look at the back of the truck. Have someone hit the foglight button and watch the tail lights. Looks like your brakes are on. All of them have rear fog lights, so you will have one fog light button. They only come on when you hit the switch and your lights are on. They turn off when you turn off your lights.

This is another reason I like my D1 better. All I had to do was buy a switch, plug in a relay, and boom, factory switch for my fog lights. Wiring was there and no ECU programming

 

I am not sure if it is different on a D1, or if I am misunderstanding you, but the rear fog lights are not where the tail lights are. I have an '02 SD, here is the light pattern.
The brakes light up the vertical lights (upper lights), whereas the rear foglights are the horizontal lights down on the bumper. The switch for the rear fog lights are on the left vertical switch panel. If you look at the bumper, there is a spot for the front lights. I know on BPUtah you can get the two light kit, with the switch, harnass and installation manual (because I was looking into it), but it is like $350

 

Rear fog lamps on the D2 are in the rear bumper - there are light assemblys with red and amber lenses. The amber lenses are the turn indicators, the red lenses are the rear fog lamps.

 

and the Fogs are for FOG ONLY!!! NOT heavy Rain!!!!!!!!!!!..........that blinds the guy behind you!!