Is the bass sounding like its far apart and blurry and coming from outside the car? and the vocals sounding like its not coming from the center at certain frequencies?
One thing you can do is pan the speakers left, then right, then center and listen if the bass is cancelling itself out? If it is, it is wired out of phase.
You can also buy a sound phase meter or go to any audio shop to check each individual speaker phase. If the speakers are 2 way speaker with tweeter and mid speakers, you got to make sure they are both in phase for both sides to ensure the crossover frequencies are not cancelled out.