The question was about the panels rear 1/4 panel were the wagons window was.

 

Well, whatever OP meant, some thoughts have been posted.

 

I've measured it, you could put a 10" shallow woofer in a custom enclosure in that area, to 'fill it up'. You would need to add some kilmat to the inside of the quarter panel because I think that it would likely buzz pretty bad.

 

The farthest rear window. Like an AA5.

 

Thanks for clarifying. I like the idea. It would take pretty shallow speakers.

Mongo's right about the panel resonating. I might try stiffening it first with epoxy, 1/4" foam or end-grain balsa, and frp. Go crazy and make it vacuum-bagged carbon fiber.

Then of course you'd need to fabricate a custom cover panel. I imagine maybe the whole thing could be designed to be reversible.

 

Tape it off then lay down fiberglass. They won't do much back there though.

Better off going out the rear windows so everyone can hear your music better