u can tap into the existing wire harness by either splicing the wires or looking for an adopter off amazon or ebay which will let u convert ur factory harness to pioneer adopter. doing either will not degrade ur sq. doing the later will not leave any tell tale marks should u need to put the factory player back in.
and i would suggest u use front and rear rcas separately cos then u will have more control on the sound and sq might degrade if u use 4 channels with a single rca pair