No Sound Coming from Subwoofer?

I have a 10" Kicker comp, Alpine mrp-500, and a sony cdx l600x headunit. Whenever I hook the three correctly I only get sound out of my four main speakers. The power and remote wire work fine because the amp turns off with the headunit and it also gets power. I double checked the RCA cables and they were matched up correctly. I even switched between two speaker wires for the sub but neither did anything. Does anyone know whats up? Do I need to buy something to fix this because all these are brand new except for the headunit. Thanks