Correct way:
Rockford owns a small division called InstallEdge (
www.installedge.com)
They make OEM amplified line out convertors. They do what the last guy said, convert the amplified sound to RCA's.... If your amp doesn't have a crossover - get one. Remember the amp is feeding off the same signal as the speakers, so if you turn the bass down on your head your doing the same thing to your sub (Thats why aftermarket head units have no fading sub outputs). Anyways that would be the easiest way to add a sub to your car.......
Also depending on sub size/amp power you may notice distortion in your rear speakers, you may want to disconnect them - you'll notice anyways that if you do disconnect them and have the sub going, the air pressure from the sub alone will cause the cone of the speaker to move up and down as if it was powered....
Chris