The rears in a 300TD are mounted straight to the sheet metal. Obviously, the stock ones drop right in (er, up), but aftermarket ones need to be relatively shallow. Put down a sheet or something when you drop the rear panel because the foam used between it and the speaker area has almost certainly deteriorated and become a sticky mess that's annoying to have to clean up.

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Joel
1985 300td (sold)
1981 280SEL (4-speed)