I've tried 4 speakers in my back door and there is a no bass coming from them. I used the old speaker as an adapter and screwed the new speaker to it so it sits flush with the metal. I've used dyno mat to the back of the door and even stuck a sheet over the adapter before shoving a speaker through it. I've tried adding foam tape to the underneath of the speaker too. My focals up front have loads of bass but my rears suck unless I put the original thing back in. If anyone's changed there rear door speakers, can you fade it fully to the one side and compare with your front speaker. The only thing I can think of is the speaker isn't sealed properly in the door even though it looks fine.