Well, I think I have shared this with you before, but to give it to everyone, have a look at this...
http://www.sounddeadenershowdown.com/You will see big differences when placing it near the speakers in the doors, it gives the speaker something to work against, rather than just flexing the metal... as far as the rest of the car, some in the hatch would help some for sure, your subs fire right into it... Like I said that day we did the fogs, my coffee can analogy.. "Take a large empty coffee can and tap on the side of it with a pencil, then fill it with water and do the same" By adding the deadener you lower the resonant frequency of whatever panel you apply it to through "mass loading" the panel will still viberate/resonate but at a much lower frequency.. lower frequencies are NOt directional and much harder to localize than higher frequencies... 1 layer is usually good for 80% of people doing it, it will change things a lot, but, as you could see with my car (which STILL isn't quite done being deadened) is a real PITA to do... The more you lay down the better the effect... Dynamat used to say one layer was good for -3db of road noise cancelation, 2 layers was good for -6db and so on... which is GOODI'm still working on the guy from my audio forum about the cheap(but really nice) sound deadener he's wanting to offer, want me to check into it for you? I'm still trying to get out of him how much is on a roll (he's really busy) but he told me something like 40$ for a roll, but again, I need to find out how much is on a roll and make a compairison of price/sqft with other products, right now, the best bang for the buck is RAAMat from RAAMaudio.. I've got it BAD lately, i've started "Great Stuffing" the car now... Lol...