I have a 12-inch Polk in the trunk of my W124. I have thought about adding ports to the hatdeck, but honestly, the bass inside the car is pretty solid as is. It's definitely louder in the cabin than standing outside the car (I'm always amazed at how well the sound is isolated inside these cars). I don't listen to tremendously bass-heavy music, but what I do listen to gets a really nice kick with that sub in there. I suppose if you wanted some really serious bass you might want to mount something on the deck or maybe port it out of the trunk like tbomachines said.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)