Your backup lights are controlled by the rear SAM (in "back seat" area behind right side seat). The signal originates in the floor shifter (no switch on trans). The floor shifter (identifying when the shifter is in R) adds the signal (that controls the back-up lamps) to the CanBus (no individual signal lines) ... passing though the ignition switch and then back to the rear SAM (where it is decoded).

Short of the unlikely scenario where both wires have bad connections (at rear SAM output .... connector #4 pins 28 and 30), you likely have a problem in the rear SAM (decoding) OR the floor shifter (encoding, or shift lever position sensing) .... You could see if there are any other anomalies that might point to the floor shifter or rear SAM, but you likely need a dealer level diag tool (star tester or similar) hooked up at this point.

Hope that helps,
Chris