Yah the MP3 system (with the h/u 02MGGT posted) is the 4 speaker sound system. I believe it was available on the standard and deluxe v6 models starting in 02 went till 04. (until the shaker system began in the new 05 platform)
It's not a bad system by any means and actually is way easier to splice aftermarket amps and subs into since all the p.i.t.a. Mach 460/1000 wiring isn't everywhere.
Here's a site with some Mach systems info:
Mach 460 Information
12 Black GT500 SVT PP l 2.6 Pulley/Idler l Airaid l o/r X l 305 NT555R's l Recaros l
04 Azure Blue Mach 1 / 03 Sonic Blue 3.8 / 14 Sterling Grey F150 FX4 402A 5.0 / 11 GT500 R.I.P.