I am going to be upgrading my Stang to a Roush Front Bumper and a GT grill. My car currently has the Pony Package grill. Will I be able to use the wire harness and switch from the Pony Package or do I need the GT wire harness and switch if they are not the same?

 

You should be able to

 

Yours work. i did the swap perfectly. Your PP harness as an addapter on it to fight the smaller bulbs. Take those adapters off and the remaining harness is the one that fits the GT bulbs.

 

Now to just address the wiring for the Roush fogs in the bumper.....