I would check to make sure the terminals (including the wires that go to the speaker cone) on your speakers aren't touching each other or the frame of the speaker. If your speaker has a metal frame, I bet you have a grounding issue. You may want to try putting some sort of non-conductive material behind all parts of the speaker's frame that touch the door (I had the same problem in an old car and eventually ended up running my own speaker wire)
