I used dry ice on my sprinter! Removed every last bit of deadener... had to do some manual scraping on the gooey bits though...

SupaGas sell the stuff for $8 a scoop (about 1.5 to 2kg in a scoop i reckon). I used three scoops, 1 to try the stuff one day, and another 2 to make sure i could finish it off. If you get more, you can probably reduce the time taken (more stuff gets colder, quicker). They also sell the stuff in bricks... a couple of bricks might work well, specially for verticals, not easy to pile up pellets on the firewall....

Chiseling woulda killed me!