Does anyone know what this small rectangular plastic knock-out piece is for that is on the left side of the rear "bumper". I also have a Chevy silverado 1500 pickup truck that had an access similar to this which is necessary to access to insert the piece that operates the raise/lower wire rope ratchet holding the spare tire up under the truck. The spare tire for the XB does not have this feature and is stored inside the vehicle so what is this knockout for? I did not see anything behind it underneath that gives me a hint for its purpose. Does anyone know why it is there?

It is for towing. There should be a bolt with an eye at one end, it treads into a hole inside the cover and they use that to pull you onto a flatbed.

The bolt-hook thing that goes in there is attached to the underside of your spare tire cover.

Thanks guys for being very informative. I guess if I read all the literature for the car I would have known what this plastic knockout was for.