I have an '07 so I'm not a 100% sure but the simplest solution would be to wire the headlamps to come on with the parking lamps via a relay harness. This would trigger the headlamps to come on with the parking lamps so the auto function would work normally and you'd be able to turn on the highbeams with the headlights staying on. The main problems are the fact that you could never run with just the fogs on (since the parking lamps are on with the fogs so would the headlights) and the headlights will blink with the parking lamps when you lock and unlock your door with the keyfob...that's not a problem if you are running halogen bulbs but if you have HID's it'll fry the ballasts. I did this for my buddy with an 02 Tahoe running halogens, works fine.
The other alternative would be to modify the procedure for running the fogs with the highbeams on simultaneously.
