Another popular way to connect an amplifier to a factory radio is to use something called a line output converter. It connects to the factory radios speaker wires and converts the speaker-level signal to a preamp-level signal. This lets you use RCA cables to run that signal to your amplifier.
How do you hook up a RCA adapter to a stock radio?
0:126:03How to add RCA Pre out adaptor to a factory car radio guide - YouTubeYouTubeStart of suggested clipEnd of suggested clipAnd its part number pc2 so one and what that is is basically its got speaker wires on one end withMoreAnd its part number pc2 so one and what that is is basically its got speaker wires on one end with an earth and RCA connectors on the other end that you can plug your RCA cables into.
How do I connect my subwoofer to my stereo receiver?
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.