It appears that the read screen receives some command from the head unit over the CAN buss to turn on when a DVD is selected. There is also a second input in the rear of the car which can display on the rear screen as well. This may go via a the head unit or it may be a video input on the rear screen which is just selected from the head unit. I need more time with the circuit to nut that one out.

So I will set up a raspberry pi with a CAN buss adaptor to interface to the CAN bus and then log some data and see if I can work out what turns the screen on.

Im using a Rasp pi 3B+ and the MCP2515_CAN module.

The MCP2515 CAN module out of the box requires 5V to run but the Pi IO is only 3.3V. A slight mod is required to the board to allow the CAN driver to run on 5V and the interface to run at 3.3 for the pi. the path I followed is as per https://forums.raspberrypi.com/viewtopic.php?t=141052 The following image is from the same thread

I found the pi would not reliable start with the 5V line connected to the driver chip. Ill look int why one day but in the mean time have found it works if I disconnect the 5V line start the pi  then connect the 5V to the CAN driver.

This is a bit clumsy as I dont have a portable screen for the Pi so I connect the pi and MCP2515 module to the can bus - For a previous project I soldered a couple of wires into the radios blue tooth module and routed them out through the glove box for easy access.

I then SSH into the pi from my laptop and run CANutils on the pi I then scopy the files to my laptop - I should work out a better way to do this. A USB CAN adaptor sounds like a good idea...

192.168.1.25 is the Pis IP address.