I have a 2002 Chevrolet Cavalier which I am installing an aftermarket radio, but the wires running out of the car, do not match up with the aftermarket radio's wires. I have cut the wires on the factory radio wire harness too short, so I was going to wire up the corresponding wires, but ran across multiple problems. There are two black wires which I am guessing are both ground wires? and a purple wire, which I have Googled into being a BCM wire? Also, there is no 12v accessory wire coming out of the car? How could the stock radio have worked without always being on and draining the battery? Could somebody please help me with how I can wire this or something I could buy to complete this aftermarket radio installation?