I had Roadkill installed in my doors when I upgraded the speakers. I used the leftover to do the trunk. The stuff I used is awesome. You don't have to heat it and you don't even have to clean the surface unless it is encrusted with dirt.
Here is a link:
I got it from my local installer. I just got my WRX and had been driving a Lincoln LS for the last few years so I was desperately in need of something to reduce the road noise--this stuff works great.