Venting is important, especially in Mercedes. The fuel tank really absorbs most of the bass. In my w124, I removed the rear parcel shelf and removed a lot of the cardboard lining. I don't remember exactly, this was 10 years ago. I think I carefully used a hole saw, enough to score it for easy removal. I may have even drilled through the metal a bit.

Drilling some holes through the first aid compartment is basically the same thing.

Not to poo poo your idea, but if you want any bass to make it to the front cabin you should get a 12" sub for a car (4 ohms) home stuff is usually 8 ohms. You will need at least 300 watts rms to get any meaningful sound.

I had a 1000 watt setup, it basically made all my mirrors useless.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2