Hmmm, it's been a couple years since I've had the door panel off, but I don't recall seeing it in there. To the best of my knowledge, it was a seperat set of wires that piggybacked off the lower door speaker, i.e. not part of the main harness.
I wouldn't trust my memory of the subject however.
ive ripped the door panels off my 01 GT (non premium system) and my 94 (mach 460)
Your GTS will not have the wiring for the tweeters, or the mounting bracket deal.. So you must obtain the whole tweeter assymbly, and simply splice into the lower door speaker for a signal.. Then you have to grind the door panel out, for the tweeter to even fit.. I yanked the tweeters from my 94, and the mounting pod deal, and swaped them onto my 01.. Works perfect, and makes a much deeper, thicker sound to the music.
You will also have to get some sort of push pins, or fasteners to fasten the tweeter to the door.. I'll show ya how to rig this up if you need.
Heres the swap i did on my non premium sound system GT.. (same exact setup as your GTS)