How do you hook up a subwoofer to rear speakers?

Some (many) powered subs have high level inputs, so you can just tap into the rear speaker wires. If your car has a factory sub, use its speaker wire instead. If it doesnt have high level inputs, you can use a basic line out converter that ties into the speaker wires and turns them into a rca output.

How do I connect my subwoofer to my sound system?

Using a subwoofer cable (RCA cable), connect the subwoofer to the AV receiver by running the cable from the receivers subwoofer output to the subwoofers line input. Klipsch subwoofers can be connected via the LFE channel, the white RCA connection, or in a stereo pair by using both the white and red RCA connections.