The RCA cable just relays the L/R audio signal from the preamp outputs on the head unit to the inputs on the amp so the audio signal reaches the subs.

Scosche puts out a very good amp wiring kit that can be found at Wal-Mart, typically ~$20 for the 8-gauge kit and ~$30 for the 4-gauge kit. How many watts is your sub amp putting out? You probably only need the 8-gauge now but if you plan to upgrade (more power, second amp) in the future, you should go with the 4-gauge kit so it's less work to do later.