I removed the interior door panels and stuffed fiberglass in them. This made quite a difference. I also began stuffing fiberglass under all the plastic panels in the back of the car. I rode in the back seat when when my son was driving once and there is more noise coming into the cabin from the rear end than from the front. I plan to stuff more fiberglass underneath panels in the trunk and back seat.
I also hear the dynamat works well. I want to try some of it underneath the car. I am not sure it will hold up well to the elements down there but I may try it to see if it even works to deaden sound from the floor area.
I did the floor, doors and rear wall in my 2008 Tacoma. So far I've only done the trunk in my WRX. Once I'm out of my apartment and back in a house I plan on doing the same. I use Noico deadening products.
I'd start reading here. There's only so much Dynamat and using just one product will do. You'll want to find out what you're trying to do as far as blocking low's, high's or all around. Low frequency is the hardest to block as they travel through thick objects. For vibration control Dynamat is popular but if we're talking exhaust noise and road noise that aren't high pitched you'll need to layer different materials. Even using an app with a frequency analyzer would help to get a decent picture of what you're dealing with.