Dynamat only stops weak metal. You need to add foam to stop the plastic from vibrating.It's just a pain, sounds like you got the screws etc. holding it I also removed the seat belt bolt downs (not sure if needed) but I don't need them so set them aside. It's a very tedious procedure one side a bit at a time then one side comes out first, very easy to break where the plastic gets thin down beside the seats trust me I know when one went snap I snapped lol easily hidden but still pisses you off. Just the center brake light has to removed as well there's a bolt you get from underneath, do a great job the first time so you don't have to do it all over again it sucks, I removed mine due to severe vibration from sub so dynamatted it but still vibrates so trying to decide what to do now. Good luck.
I have a 12" in a 2.0 cubic feet after displacement. When I turn it up even I can't hold it and stop the rattle noise. I heard closed cell foam and the spray on foam can stop it. Also are you on stock battery?Yeah a buddy mentioned that to me but with just the little American Bass dvc with 1000watts going to it literally lifts that plastic high enough it touches the rear window, because of that it rips the little push tabs on each end that are to help hold it in place they pop right out. I'm planning on adding a 2nd AB 10" or a 12" dvc sub yet to be decided but it's going to have 2000watts rms & 4000watts peak at 1ohm going to the sub/subs so I've got to come up with something decent without having to cut, drill, modify the factory piece's incase we decided to ever sell. I wanted to use some kind of nut & bolt design with some way to hide the heads on the plastic and tighten it down onto foam maybe Styrofoam in between the metal and plastic to stiffen it so it can't vibrate. I've got a few months to figure something out, hopefully something simple and non damaging to the car because my wife would probably kill me she hates the stereo now I can imagine what she'll be like when it's shaking more stuff then usual next season! Lol