Have you tried playing with the settings of the stereo? Fade/balance and bass/trebble to improve the sound. Also are the speakers in the door stock or aftermarket? If they are after market its possible that they have the wrong ohms specs.
98 GT conv. PI intake, MHS stage III ported PI heads/Stage 3R Bullet Cams,
JLT RAI, 90mm MAF, BBK 78mm TB,
LT headers, Mac o/r prochamber,
FRPP underdrive pullies, FRPP 4.10's,
SLP LM1, built 4R70W 4000 stall.