The 'system' (especially speakers) the previous owner put in were garbage (ebay crap, cheap as hell).

Your first thing to figure out is whether or not he bypassed the factory amp under the passenger floorboard. If he did, you will have to rewire if you want to use a factory head unit (because there is no onboard power). I believe he bypassed the factory amp since the original speakers were subs only capable of up to maybe 150-180 hz and you have full range speakers in there now (can anyone confirm if there are frequency cutoffs in the factory amp?? I have an aftermarket one so I am unaware).

I would recommend doing more homework as to how everything is CURRENTLY hooked up and go from there, I am sure based off the responses that there are many who are willing to help.

John