Re: sound deadening
Dyna mat like material does not deaden sounds, it changes the frequency to a lower range, killing rattles and vibration noises a bit. It is solid and can not adsorb sound like fiberglas mat does - slowing the air motion thru friction.
I used it ( dynamat ) to stop the oil can sounds on the roof of my car making its response much lower pitch and less annoying. Its a small car and your going to hear the rear tires and exhaust, to improve the sound pressure level, you would need to add alot of sound adsorbing material. I dont believe this can be done in an appealing way.
One thought that I had was to add a plexiglas window between the trunk and cabin, that would help as we cant access the back anyway unless its to throw a ball cap back.
Enjoy, Woody