Those door speakers can really buzz if you don't add a pair of capacitors as a low pass filter.
The stock stereo routes freq's to the door speakers that they cannot handle. A low pass filter will let them be mid-range speakers, and let you boost the bass to the subwoofers without that annoying distortion.

Check out the "best stereo bang for the buck" thread.

It's a simple mod and really cleans up the sound.