Hey, nice mustang by the way. Mine is a '98 GT and it has the same interior. I fit 0/1 guage in it fine. Mustang interiors are extremely easy to take apart, everything just pops off. So Im sure you shouldnt have any trouble.
Ive been through like 5 setups in less than a year. Ive had atleast 6 decks, and changed my front speakers for the second time already. I have a beautiful RE HC 15" sitting in my living room right now waiting for my Kove amp in the mail. Anyway, what Im trying to say, get the lowest guage wire you can afford because you never know what you will get next. My last MTX amp fit 0 guage.