[Tireiron]

It's part of the headunit iteself. The CD unit is only a CD. And there is no external amplifier. If the speaker works correctly when connected to a different souce, and the wiring connection to the speaker is good in the door (test the wiring from the speaker to the headunit to be sure there isn't an issue with it, could have become damaged somewhere), chances are there is an issue in the headunit.