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.
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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