There are two different tailgate harnesses according to the parts catalog, one with the camera plug and one without. My LTZ (no nav or camera from factory) did not have any camera wiring or plugs in the tailgate, had to install new wires for that. I verified the wiring using the wiring diagram, yup - everything there except that.
It's not too difficult, except for getting that RCA plug through the grommet between the body and the tailgate. Without silicone spray or something else that's slippery (and does not degrade rubber) it is absolutely impossible to get a cable like that through.
EDIT:
When you remove the inside panel of the tailgate, look for a connector that looks like this:
(picture from someone else, found it on Google)
It should have six wires: white, grey, dark blue, light blue, black, pink.
If you have this plug, then you have the wiring for rearview camera.