Extra Second Skin?

[CBFryman 2]

Since soon I will be installing a new sub in my truck (if i can ever get in contact with nick at IA) so while i have the 180lb behemoth out of my truck i figured I'd slap the leftover second skin somewhere. Im looking for the most quiet and the most potential SPL gain.

My doors have 1 layre on the door skin and 1 1/2 on the door (didnt botehr putting a second layre over the window acess gap incase i ever need to acess the window) I have 2 on the back wall and that helped a LOT. It went from slapping my bedliner to maybe 1/2" of movement full tilt. I feel much more comfertable upping my SPL now that my back window isnt dancing...as much.

The floor and firewall would probably be the next place to start correct?

Or would the headliner be better? If i do the headliner i will also lay some overkill in up there.