The best way to connect it to a factory system is to tap into the stereos speaker outputs for the amps input signal. Then send the amps outputs back to the stereos harness, and on to the speakers through the factory wiring.

How do I set my RCA to factory 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.