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.
What is a factory amplifier?
Car audio head units in all basic factory sound systems have small, built-in amplifiers to power a handful of speakers. Meanwhile, premium factory-installed sound systems usually use larger, more powerful outboard amplifiers that are separate from the head unit to power more speakers and achieve better sound quality.
How do I know if my car has a factory amplifier?
0:415:14How Do I Know If I Have A Factory Amplified Audio System?YouTube
How do you bypass a factory amplifier?
2:234:10How Do I Bypass My Factory Amplifier? | Car Audio Q & AYouTube