I had a similar problem with my old car. It as a midsize Ford. The pressure coming thru the rear deck was emence because of my 2 15" W3'*... and my roof bould buldge... If I turn off the radio you could accually hear the sheet metal go pop when going back into place. I gutted my back seat and wraped the top fabric on top of fiber glass... and cut holes in the the glass to let the sound thru... so the sound pressure could push thru the seat and not put so much strain on the roof because of the rear deck. The good thing was I could still have people sit in the back... I used fiber glass just because it held the seat together with foam and all cept where the holes were... the steel cage just wouldnt work. And I didnt want to take the seat out completely because I had people ride with me often.