How to connect an OEM stereo harness to a 2006 Chevrolet Cobalt?

I tried installing an aftermarket radio into my 2006 Cobalt, and after realizing that was a mistake, I am now trying to reinstall the stock head unit. I bought another OEM harness, and tried rewiring everything according to a diagram I found, but nothing happened. I tried connecting the red/white wire to the OEM yellow wire, which is supposed to power it, and nothing has happened.