If you are looking for a moderate bass boost, but do not want something that
rattles your teeth out, I would suggest looking at the Bazooka Tube line of
subwoofers.

http://www.sasbazooka.com/

Your best buy is probably one of the units with a built-in amplifier,
meaning you only have to run a battery wire and a remote turn-on wire from
the dash to the unit itself.  Then it's just a matter of wiring two leads
from each rear speaker + positive wire to the units inputs.  Some models
also come with a remote controlled volume control so you can control the
output of the subwoofer seperate from the rest of the stereo, right from the
dash.

There are alot of these units on Ebay for decent prices, so I would check
around before I buy.

Good luck
Sharky