Firewall.
Toeboard.
Floorboard.
Cab corners.
Underside of the cowl. A lot solar heat radiates through the cowl.
Inside of the outer door skins.
Ceiling. I use rubber compound cement on the ceiling to make for better adhesion of the stick-on roof flashing that I use.
Makes a very noticeable difference. You don't have to yell at the Ole Lady for her to hear you, but you still won't be able to hear her mumble to you.
