Are you using one of those line converters to install a sub? If this is the way you're gonna go, just get a line converter. It's simple to install, the line converter takes a signal from the rear speakers, just connect the positive wires to positive on the speaker and negative on the negative, the positive should be the larger terminal on the speakers. Both speakers need to send a signal to the line converter. Then from the line converter you just use RCA cables to connect to the amp. Once you've done this the rest is not hard to wire, running a direct power wire from the battery to the amp with a inline fuse then setting up the levels on the amp, but you should set the bass on the stock radio to full negative with line converters. But on aftermmarket decks there's a Sub woofer output that can be set on the radio.