The odd thing about this is that it is always the right rear..I had the same problem and the right one is all rusted out and the left one is near perfect.

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/car-audio-multimedia/125099-123-install-extreme-dynamat.html