The protector/air deflector goes from just behind the bumper skirt, back perhaps 2 and a half feet - if you can see into the back of the bumper then yes, you're missing it. Well, either that or I'm missing the same one as you !!!!
I'm not sure i'd be doing that, the areo on the car is very carefully worked out and if it would have been a benefit i'm sure GM would have fitted one. I suspect that if you fit one you will disrupt the air exhaust from the radiators.