An easy way to hook up the front components is to: 1) cut all of the speaker wires behind the head unit and connect the front right to the rear right and the front left to the rear left, 2) take the rear speaker wires from the rear dash speakers and hook them up to the amp as the front two channels, 3) then run new wires to the rear speakers.
I like this method because it is really simple, basically you are running a long, cheap wire to the front doors without getting through the doors with new wire. Obviously a newer aftermarket wire will be better quality, but it will work fine for now and you probably won't notice that big of a difference.

Now that I look at previous posts, listen to Bangin, he is pointing you in the right direction. Only do my method if good aftermarket wires aren't already there.