Car audio!!!

I've wired systems into cars. But now at the time to do it on my new car. I don't have the money to buy a new head unit. So my question is on my 2008 grand prix. Do I need to buy a head unit? I have a duel system Two 12in subs. And the amp. Do I need to get a head unit. Or is it possible to wire it right in?
Thanks!!