Didn't want to lose too much cargo space, so a small ported Digital Designs 8" 508 woofer was perfect for the job. That, coupled with a DD C2c 2 channel amp, and the low-end was rounded out nicely.
These mini woofer/box combos fill in the lack of bass that almost all OEM systems have, and their small footprints make them ideal for lack of cargo room.