Yeah,both of my diesel trucks have that "full plastic jacket" bolted over the entire firewall...thankfully some previous owner took a hammer & chisel to that on my pickup,so they could replace the blower motor--I would be tempted to do the same thing,had it been me changing it!..that plastic cover made it difficult to put my plow cables thru the firewall on my Suburban too,good thing someone else had already made a few holes in the right spot for wires and I only needed to enlarge them..
That cover has insulation under it,and it does quiet down the noise in the cab substantially--but it sure makes working on anything in the firewall area a PITA..