My carpet in my 91 GT conv was pretty well stained and nasty. If you have the time and are willing to do what I did...take out the whole carpet!
I took the carpet out and hit it with a steam cleaner. We have one at home for carpeting and it works wonders! If you dont have that, go rent one. I scrubbed the hell out of it with some spray carpet cleaner and then went over it with the steam cleaner. You may have to do this 3-4 times, but the results were AMAZING!
I figure you may as well try instead of plopping down $$ for a new carpet!