There are two basic types of products available at the local home place. The first group is great for insulation. One is a foil covered bubble wrap. It's about 3/8" thick and needs to be attached with spray glue. Not super for noise insulation, but great for heat control. Look for it back in the insulation section. The ohter is a felt padding with a foil barrier on one side. You see this stuff listed in some automotive catalogs. Try looking for it in the water heater area. I think it is packaged as a water heater blanket. This stuff will soak up a little noise, but will also hold moisture if you park the car outside in the winter. I try to stay away from the fiberglass based stuff, although it works well in areas where it isn't likely to be disturbed, like under and behind the rear seat, and around the rear wheel wells behind the rear panels. Sealing it in place with plastic will help. Make sure the plastic sheeting behind the seat, and door panels is in good shape and sealed. Foil tape over the holes is a good alternative here. All of the above stuff is pretty light and does well to control heat and air movement, and will wipe out some noise.
There is another group of products that work great if you just want to deaden sound, and resonance. There is a sticky roofing underlayment material used for ice-dam areas in cold climates. This is like a heavy version of Dynamat. Not cheap though. Finally, there is the time honored method of black roofing felt and glue (like tar or linoleum mastic). Messy to install, heavy, but cheap and quite effective. Works great under floors, and big flat panels like door and quarter skins, wheel arches and trunk floors. Also, try the foam type household carpet pad under your trunk mat, and/or carpeting the trunk. Beware of moisture retention though if you park outside in the winter and your trunk leaks.
How about a nice 4 channel amp for your stereo?
