Radio removal
I don't know how to remove the radio however, you may be able to find some Sonic radio removal videos on YouTube. I am sure it can't be that difficult. You also can go to GMparts.com and look for the radio. Generally, they will have an exploded diagram of radio and all of the components surrounding it which should provide insight as to how to remove the radio.
PS
My Sonic is a 2013 and there were no RCA outputs on the radio so I had to go a different route when I installed my Alpine amp. You are probably going to want to change all the door speakers too because the factory speakers sound bad when matched with a quality subwoofer.