Most stock car audio systems do not have a low level output such as RCA but a car audio output inverter can create one for you. This is especially important if you are trying to install any amplifier with subwoofers and other aftermarket car speakers. Aftermarket car audio parts need more amplification, which requires a low level signal to start with. A car audio output inverter is more commonly know as a "line output converter" since the device sold isn't always used for car stereo. Any line-out converter will work as long as it has RCA output and can handle the wattage your stock stereo gives to each speaker.
Turn your car off and take the key out of the ignition. Do not power any electrical devices within the car while installing the audio output inverter.
Locate a section of speaker wiring going to a speaker by removing the car stereo, or gently removing some car panels nearest any of the speakers.
Find out which of the two stereo wires you are holding are negative or positive. Examine the speaker terminals in the back of the car audio source, refer to your manual or the car manufacturer website to find out.
Gently strip away a small section from each stereo wire with wire cutters. Only cut through the outside layer of wires to expose the metal wire beneath.
Wrap the positive and negative leads of the car audio output inverter around the respective wires that were partially stripped. Wrap each wire with some electrical tape between attaching each inverter cable. Cover any exposed metal and wrap around each wire once more together.
Replace the car stereo and any panels you removed, while tucking the line output converter out of sight. If you are installing the amp or other parts right away, then insert the RCA cables into the output converter before hiding it in the paneling.
If you cannot determine the polarity of each speaker wire, don't worry the sound will most likely be the same and no damage will occur. Distortion would only occur if you re-amp all the speakers on top of the power they already receive in the opposite manner. You can wire all car audio speakers in reverse as long as you follow the same wiring rules for each one.
Do run the car at anytime when installing anything. Do not mess with the car battery or other electrical connections before researching fully.