the big problem with the original style firewall mat, is the fasteners used are big plastic push pins. chances are is you've welded those holes closed. i use raammat, ensolite foam and carpet pretty much as high up on the firewall as i could. everything bought at raam audio, not sure they exist anymore.
i wouldn't install wires behind anything that can't be removed easily. plus rubbing between the layers may wear on the wire insulation. granted, i've never had to rework any wiring in truk. but i may be the exception, not the rule.
