Another option is to get a powered sub-woofer. Types like the Bazooka Powered Sub-woofer allow you to take wires from your rear speakers and attach them, plus a power wire and mount the sub in your trunk. It's probably not as good as the seperate amp and sub in pass-through enclosure, but it's simple.

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Steve
<UL TYPE=SQUARE>
<LI>1979 240D Josephine
<LI>1990 300E Emma
</UL>