okay, the factory H/U will be able to be used with a sub BUT....
you will need an amplifier to run the 8" sub, a cheap amp such as a Jaycar 2 x 150WRMS for around $300 will do the job, but you can get amps cheaper from perhaps strathfield.
the stock h/u will run an amp but you will need to buy a "line level to RCA adaptor" ($20 approx from marlows/jaycar/strathfield)
if you want to know more then feel free to contact me over msn messenger at
brewsta@hotmail.com or email me at
hbrewste@bigpond.net.au