The Audio Control unit I linked and the one mentioned by trdriky91 are line out convertors. They let you keep the factory stereo, while boosting the voltage level from the factory wiring harness. It makes adding an aftermarket amp easier. Getting a capacitor can help too depending on how big of an amp you get. It acts as a power reserve for musical peaks, so you don't strain the battery or alternator. Basically it helps keep a constant steady voltage output to power your system.