Problems like the OP is facing are very difficult to resolve. At one time, shifting into reverse triggered an actual physical switch that led to the backup lights. Pretty easy to trace a wiring problem or a switch problem. Now, you shift into reverse which just puts info on the CANBUS saying in effect "driver selected reverse". Then, various modules on the bus use that info to trigger the reverse lights, turn on the rear camera, change the screen output to rear view, put the trans in reverse, turn off parking brake, on and on- all while running through software and checking various saved "mode" settings (which were also transmitted over the bus) to determine whether this is action is allowable. While that's a good thing for safety, diagnosing this stuff is extremely difficult, and the software is glitchy. Luckily, a lot of times the problem can be fixed with various resets which clear certain "mode" settings that are still in the bus modules. There is a LOT of software in these vehicles.
