I saw threads on how to hook up amp to stock stereo. So here's a video that will help 2012 chevy cruze amp install with stock radio - YouTube
I can guarantee you the stock system does not deliver 20 watts per channel RMS. I would be genuinely surprised if it could produce more than 10W RMS per channel.Stock radio is pretty bad. Im thinking 20 watts per channel. If you grab a amp running between 250 and 400 watts. (between 50 and 90 rms) Your stock speakers will handle it just fine.