When I say "amp module", I mean an integrated circuit on a circuit board either in your head unit or in a separate amplifier. I have no idea what sort of car, or what sort of stereo is in it. Doesn't really matter, there is little that you could do to test such a thing.
At this point, you should do what you should have done from the start. Take the door apart and have a look. Consider swapping the speakers from left to right to see whether the problem stays put or follows the speaker. It might even just go away if it was a loose connection.
Failing that, get someone who know about wires to have a physical look at it, in person.