Luxury cars with walled off trunks are famously known as "bass traps". This has been the case in every one I've installed a sub in without making sure there some way for the sound to vent into the cabin.

I'm not familiar with the rear deck in 123 coupes, but you may be able to fit larger speakers back there and only have to cut the upholstered panel that can be replaced without affecting resale.

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