Not sure about the window tint legalities, my experience is your better off just going pro. If you mess it up its horrible to remove, and more than likely you'll end up with bubbles.
I just put some new tails on my GT of the same year. Actually had to do it twice since I forgot to put in the weather stripping.
If the GT's interior is the same as the GTS' you'll first have to remove the panel closest to the rear of the car, its where your hatch locks down.
There are 2 10mm bolts on the bottom of the panel that have your tie strap anchor things on it. There are a couple of plastic pops that come off after that.
After that panel is off the next one is the tray behind the back seats, I don't think there are any screws on there but someone had been in my car before so there may have been a couple of screws missing, mine just popped off.
After that you have to remove the 2 side panels on either side of your trunk area. The one on the right has the little light so make sure you unplug the little lamp cord.
FINALLY, after you remove all of that you will see a 3rd bolt that holds your tail light on, in total there are 3. They are all 10mm as well. Pull them all off and slip out your tail light. It's actually much easier than it sounds but it will take 30 minutes to an hour to complete. Don't forget your weather stripping! haha
Do it in reverse, and re-install your interior.
Good Luck! If you have any more questions just ask. My biggest issue was missing that last bolt that was hidden away.