Yah if you have the Mach system it will be difficult to tap in a head unit and such. My friend's 01 GT has an aftermarket head unit (already in the car when he bought it) with the mach 460 and the sound is terrible - no bass, no volume.
I believe the audio systems without cd players and just cassettes are not Mach 460 systems...don't quote me on that though...they came out with the Mach 460 in 95 and have different levels with cd players, without, with 6 discs and subwoofers in the trunk. A good way to check is whether you have tweeters up by the mirrors on your doors...
Here's a link to some Mach Audio info...
Mach 460 Information
12 Black GT500 SVT PP l VMP TVS l JLT123 l O/R X l 28x11.50Rx15 M/T Street R's l BMR Suspension l Recaros
03 Sonic Blue 3.8 / 17 Magnetic Fusion Sport 2.7 EB