I have replaced my Head Unit (MMCS) with a Pioneer SPH-DA360DAB and it works really well.
I have connected it up to the read camera as well and can manually switch to it on the stereo and it work.

My question is I want it to auto switch like it did on the original head unit when I select reverse. Does anyone know which wire behind the head unit is the reverse indicator?

I have recorded what I have done to share with everyone as it is a really awesome unit. Very happy.

 

I am afraid, there is no such wire.
According to electrical circuit diagram (for PHEV 2016), the lever is connected to PHEV-ECU and then, perhaps, the reverse signal is transmitted digitally to MMCS via CAN bus.
See diagrams are attached.

Maybe there are small microcontrollers are available for purchase, which would catch that reverse CAN frame and output analog signal for 3rd party stereos. I am working with CAN at work (for space applications though!) and I know that it would not be too complicated to make such adapters (ooopps, another business idea of mine just leaked out...)