Wiring in Chromecast (Or similar streaming device)

Hey all, I've been looking for the best way to send an arbitrary video signal to my 2017 Cruze (with full nav package and all safety package).

Given that:

• From what I can see, it looks like if the backup camera has power, the display will default to that video input.
• It seems that this can be isolated from the backup lights and placed on a power switch

It follows that I can wire in a different signal by splicing a relay into the camera's video out line.

My main question is if anyone knows where/how the backup guide and rear proximity warnings are added into the video feed. It seems like it's done in the mylink console, but does anyone know if they are only displayed when the car is in reverse or when the camera feed is powered on? If they are displayed when the car is in reverse, I don't need to worry about it (As I'll look to set up the power from the backup lights to override the relay switch). If it is tied to the camera power or the display being active on the My link, I guess that the only way to turn it on/off would be with the car's options menu, would there be a way to tie into the OBDII interface (or somewhere else) to send a command to disable/enable these lines.

Question 2 is, what is the best way to bring the audio signal to the car? my current thought is a Bluetooth module or running a 3.5mm cable to [somewhere], but I'm open to other options if they exist

Suggestions and solutions that avoid cutting into the interior paneling would be strongly preferred​, as I'd like to maintain as much resale value as possible.