Insulation between the seat back and the trunk? Do tell!! Mine was long gone and I've been trying to figure out what to use.
I was planning on using some sort of Dynamat on the sail panels at least since there was no insulation left. Where my OEM mat is missing at the front edge of the trunk floor behind the rear seat, I was thinking about making my own out of Dupont Kevlar bedliner. It now comes in an aerosol spray.
I haven't pulled up my carpet yet. One local upholstery shop I stopped in said he just uses carpet padding for the insulation between the roof and the headliner. I saw that at Lowes and Home Depot or he said he would sell me enough to use.
As an alternative, re-read this thread and see the post by mac88chp about the headliner:
http://www.specialservicemustang.net...ead.php?t=3081. I've bought a roll, but haven't got it installed because the depression needs to be removed where the antenna mount was. That means repaint the roof. So I'll wait until I can get that done.