I'm starting a complete go-through on my car, and soundproofing is high on the list. Dynamat is a really good, effective product but expensive. There are other materials that will just about do the same job but are lots cheaper. I recommend putting it anywhere there is a flat surface to accept it. I will be doing the floors, firewall, inside the doors, headliner, inside the trunk, and the undersides of the hood and trunk lids.
The foil tape idea is a good one and it really works. I did it when I replaced the carpet with vinyl in my truck. I used some really cheap insulating sheets, and with the foil tape I had the whole floor done in short order.
