do you still have the factory head unit (HU) or has this been upgraded?
are you planning to run all your current speakers in addition to the sub from the amp, or just the sub only?
if ur still running the original HU, you will need to tap into the speaker wires for the RCAs to connect to the amp, as the original HU does not have any RCAs out.
if u have an aftermarket HU, you will have to decide whether your going to run your speakers from the HU or from the amp, this would probably decide on how many channels and how much power amp you should get