I just went through something similar...when reverse was selected I wouldn't have any guidelines (new 2017 with factory rearview camera). If you have the factory trailer hitch, when in reverse you should see 3 icons at the bottom of your screen from L to R: first, the "Back" arrow, then a profile of a van and last, a profile of the van with hitch. To get the "Hitch" view you use the "Up/Down" and "Left/Right" buttons to select the van with hitch and push the knob in to select. Then the camera should switch to the correct view (wide angle with hitch guidance lines). If you don't get this, then the coding for the camera is incorrect. Make an appointment with the dealer to have them re-flash it with the proper coding.
I took me 3 weeks off "asking" to get mine done as the 3 people I dealt with at the dealership kept telling me my problem was normal...heard all types of excuses as to why I didn't have guidelines. Once the van was in for them to look at...it took approximately 5 hours for them to figure out the issue and correct it (they did give me a loaner Metris). If they know what they are doing, it shouldn't take more than an hour I would think. My exit paperwork showed my camera was originally programmed with the "incorrect VIN".
Anyhow, I hope you don't have that difficulty with your dealership.
There is no programming you can do through the Audio 15. There is no camera "set-up" menu and no secret "engineering code" you can enter (that I have found). The radio must be programmed through the OBDII port with MB software to fix the issue.