I think you'll have a hard time hearing anything from a woofer if you put it in the trunk. The low-lows that a sub puts out are non-directional and can also be felt, so a sub in the trunk achieves a lot. A woofer is going to put out frequencies that are directional, so location will be very important, and the trunk is a terrible place. You'd be better off putting component speakers in the kick panels or doors. Like RichC mentioned the crossover you choose will be important too.

My advice would be just replace the speakers with high-end components or even co-axs, and amp them. I have Focals, 4-inchs in the front and 5 1/4s in the back, and the sound is unbelievable. I have them amped and also have a 12-inch sub in the trunk. When I turn the sub off, the sound is still quite powerful. Even with 4-inch speakers in the front, it sounds brilliant. I think the quality of the speaker and the crossover, and the crossover setting, is crucial.

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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
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