I should add, this mention in the manual makes it less likely that a factory camera can be added to a car that came without it. Because now you're replacing the harness for the trunk lid. But if the wires are there all the way to the trunk, it still should make it easier to add a camera then trying to run the wire all the way to the head unit.However, the bad news is it looks like the harness for the trunk is different. In cars without the backup camera, the "license harness to rear compartment lid harness" is only 4 pin. With backup camera, it's 10 pin.
last issue to address, one guy posted he got it to work then no more posts from him.If I'm reading it right, it looks like a replacement harness for the trunk is $50 or so. P/N 95166042. The camera is about $105. P/N 95407397. (Both part numbers are for a 2013.) The real question is if the wires are there between the trunk and the head unit, and what kind of programming might be needed for the head unit.
Edit: Crud, you'd need a license plate wiring harness. Another $60 or so.
I saw it in OEM GM Parts Online | Wholesale GM Parts Online.The harnesses you mentioned seem to be discontinued. They're unavailable on every single OEM parts store I've checked online.