why not try taking the lid off, and fitting a dual 8" woofer setup that can slip in, without much if any modification except a few hold down screws?

I could have done the same, my bass can be boomy or not, its only a matter of how high I set the amp. you could always have the 12 int he trunk, and make a port for the sound, without actually porting the woofer direct into the cabin.

If you heard mine and how it can sound, based on various factors, music, etc, you would like it, but it might be on the boomy side for you.

I can easily adjust the bass amp to a lower gain setting which cuts the bass and makes it sound less boomy. I can also change the subwoofer frequencies too. all this and I can tune it for boomy or not so boomy. I prefer a little bit boomy, but a little bit not so boomy, so I adjust mine all the time.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon