The best bang for the buck is to use a blanket type insulation such as "The Insulator" from Aircraft Spruce. Pull out your seats and carpet, lay The Insulator out on your floor from as far up under your dash as you can get it all the way to the back window. Cut out a bunch of fitted pieces, so you have a solid sheet of the stuff from the dash all the way to the rear deck and from door sill to door sill. Now, tape the seams up with that aluminum ducting tape. Use the kind with the peel off wax paper to seal up all the seams and tape the edges to the body.
I do behind my door panels and under my headliner too. If you can hear rattling/resonation on your body panels when you tap on them, you can deaden that noise with a product like Dynamat Extreme. That is about all it is good for, sound deadening of resonant panels. It is not a good full car vapor barrier like properly applied blanket style insulation. Bulk packs of the stuff can be found at Amazon for about half price with free shipping, which makes it viable to use.
Once you have cocooned the interior with the stuff, you will be amazed at how much quieter and cooler your interior is.
