Just track down where the speaker wires under your seat are going. If they are running to the factory harness to the speakers, use that if the factory speaker wire doesn't look TOO pathetic. If you care to spend the time, running fresh new wire is always better.

I'd say just reuse whatever the previous guy had going. It'll be easier and should work.

Make sure though, if you use the factory speaker wire from the radio harness that you cut the headunit's speaker leads. This should be a no brainer but, I've seen it done before. I've also seen people run the amplified speaker leads into the radio before haha