I can't seem to find anyone who is willing to install an amp and subs into my 2004 GT. I've been told that the proper System Interface Adapter isn't out yet, and won't be out for at least another 2 months...Is this true ??? Has anyone installed a subwoofer and amplifier without making the rest of the speakers sound like garbage ???

 

You can try this to put it in. http://www.mustangworld.com/ourpics/fcar/despeeds1.htm however, the 04 radios are different because of the Satellite radio option in it. Although wiring should be the same. Should only cost you about 20-40 for a wiring kit and about 15 for the RCA adapter.