If you put it in a minimal-size box you can play it right through your rear seatback if it is reasonably acoustically transparent, and not eat up too much of your trunk space.

Actually, your seatback doesn't need to be very acoustically transparent at all. My Honda has very good insulation between the trunk and the cabin, but I just crank the subwoofer to the point where I can hear it anyway. And you'd be surprised how quiet and non-thumpy the car sounds when you're standing outside of it. In fact, standing outside the car with the Empeg playing at 0db and all the windows up, it sounds evenly EQ'd, not "all thump" like many cars I've heard.

It helps if you're driving the sub with proper amplification- mine gets 300w RMS and the input gain on the amp is near the 50% mark, and I can hear it from the trunk just fine.

There are exceptions to this, of course. For instance, the gentleman who wanted the sound from the subwoofer in the trunk of his limousine to reach the driver's cabin. This is a little much to ask (although I think he was able to come up with something satisfactory anyway).

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Tony Fabris