I know my old tv only had one also. I just bought a 2 to one and connected both the RCA cables into it and plugged it in. (Don't know about car radios though.)
Mine has one red one white; they are connected at the amp like they are at the head unit. The REM is also connected. And the power source is hooked up. The indicator light on my amp is on. The sub is hooked into the amp also. Still nothing is happening. P.S. I still have one unused RCA cable that came with the wiring kit.