My xB has it on the floors, both sides of the doors, roof, hatch, fender wells (Inside), and any interior metal panel I could get to. I have always combated engine/wind noise with louder music though. Now as I recall behind the carpet on the firewall is either a rubber type mat or an insulated pad which is an uber pain in the ass to remove. And once you commit t it, you have to get it all off or else the mat will not stick to it.
Here are a couple of pics of what I mean.
