When I redid my whole car audio system, I added sound proofing as well. I started with Dynamat Extreme - and when I ran out, I switched to Cascade.

I bought the DE online and got a pretty good price for it (can't remember where, do a few Google searches and you'll find some good pricing). I bought a whole car kit, and barely had enough to finish my trunk (I covered the trunk really good).

I switched to Cascade because the DE was fairly messy. Cascade was easy to work with and seems to do a very good job.

I bought Cascade online at an ebay site.

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1995 E300D - 225,000 miles - White Exterior, Grey Interior - original owner