Absolutely everywhere you don't see it or don't care (inside trunk I don't care or could hide it various ways)
Even under the headliner.....behind door panels. ...everywhere.
People will say "it's heavy..." - add more power.
Add more power? That might offset the advantages of the "free stuff"... Speed costs money, son. How fast do you want to go? :p
Is that sound or thermal insulation? I don't have any in my Dart (except for new front carpet from ACC, and the stock headliner with fiberglass pad) and it's really loud inside, even with tailpipes out the rear bumper...