Line Out Converters.
Will allow you to retain your factory headunit, but all give you RCA outputs for a subwoofer.
You can purchase them online (ie.
http://www.mobileelectronics.com.au/store/...t&catId=11) or any decent car audio specialist should carry them (Northfield Moorooka, Audio Express, Car Tunez, if you're in SEQ)

ROCK ON
R~R