I have a 10 inch Infinity Perfect subwoofer in a properly sized truck sub box. These boxes are designed to fit behind bench seats. They have a wedge shape and take up the least trunk floor space. I removed my first aid kit and plastic holder to allow better sound travel into the passenger compartment.
If you are thinking about using the first aid kit opening to mount a subwoofer you would need what' s called a free air subwoofer. In essence your closed trunk compartment becomes the speaker box. Free air subs are designed to perform in this environment not in sealed or ported boxes with specific volumes.
Again I just tinker with this stuff.
I've found the Crutchfield and Rockford Fosgate websites to be good sources of information as well as various car audio discussion groups.
If you're planning on getting a sub you'll need to wire a seperate amp to power it .

Oh yeah, exit 102.

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72 350SL #69
73 450SL Parts Car?
78 450sl Parts Car
79 450SLC - Sold
1985 280sl
87 190E 2.6 - Donated
90 300E-Junked
97 E420