Following my horse stall mat installation, I hear near-zero noise from behind me. What noise I hear now comes solely from in front of and beside my head. Wind noise from the mirrors enters via the door glass and the B-pillar, other wind noise enters via the windshield.
Motor noise is the other contributor and the only way to reduce that would be the relatively inaccessible firewall.
Other than fuel, my 12VDC fridge and two lawn chairs, the horse stall mats were the best money I’ve spent on Frito. Less than $100, IIRC.