Actually I've had all the audio in my car replaced except the rear sides.
The doors in the stock GT and GTS have 2 tweeters (the little speakers on the top of the door) and 2 6 1/2 speakers at the bottom of the door.
That's 4 speakers so far.
Now the rear sides MUST have 2 tweeters and 2 6 1/2's just like the doors to add up to 8
I think maybe in the GT doesn't have 2 tweeters in the rear sides... but that's just a guess