What you are looking at is MLV. Mass Load Vinyl. From what I've read, sounding should be looked at as a system. What I did first, was do use a dynamat type material with about 35% coverage, directly on the floor, toe board, and firewall, then the rear of the cab. This material does not have to completely cover, it their to primarily reduce/eliminate harmonic vibration which creates noise. This was followed by using a two sided aluminized closed cell foam material, which I used to completely cover everything I could reach. This is to cut down primarily heat/cold transfer. It's easy to work with. This is followed by MLV which is weighs about 1lb per sq.ft. It's very dense and very heavy. This material is used for sound proofing home theaters. It will reduce noise migration. It's stiff and doesn't like to bend in multiple directions, so I ended up using as few pieces as possible. This was place on top of the aluminized material. Of course I followed that with carpet padding and then carpet. So in my case it's a 5 layer system.
If you'll google search MLV, you'll find several articles on the subject.
Glad to hear that your doors sound much better. I still need to tackle my doors.
