I'm curious about why the trunk of a W123 is so much 'tighter' than the W126. On my '85 380SE, if you lifted out the plastic tray for the first aid kit, and'or the storage tray, you'd be looking right at the top of the gas tank. There doesn't seem to be anything except the carpeted panel in front of the tank separating the trunk from the interior. And since the plastic of the trays is thin, and the panel in front of the tank isn't anywhere near air tight, maybe that's why my Kenwood sounds fine from the trunk.
Does the W123 have a steel panel under the tray, separating the trunk/tank?

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1986 560SL
2002 Toyota Camry
1993 Lexus