I believe that the 3rd gens have a European style headlight system, where it's a left & right separate system and all goes thru the instrument cluster. I was trying to install an automated light system but I'm not too sure how with these trucks. So hopefully some one will chime in that knows these systems on our trucks
On my '06 my foglights go out when I hit my highbeams, with not looking at a wiring diagram for my truck yet, to me its like they are unswitching the power through a relay that supplies the fog circuit. The hi/low lever is controling the function of the 3 circuits.
Are you adding "additional" foglights to your current fog system, or new foglights ?
I do know that the headlights go through the dash cluster, whe I was changing the gauge cover, my headlights came on while I had the cluster out of the truck. Other than wiring a separate toggle, I don't know what else you can do. You might want to try to use a relay set-up that gets triggered from the headlight wire. All depends on how you want the lights to work.
I used the power to my Hellas to power a separate relay that then powers the fogs. That way I have fogs with my low beams, and fogs+hibeams+hellas on high. Works fine with no problems in over 5K miles.