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As far as the software is concerned, this is the “mounting plate”:
It takes 3V3, GND, I²C SCL/SDA, and USBD+/-, and passes the rest of the pins into the (snipped) cable. Over USB, it presents the SD card as a mass storage device, and over I²C, it presents a 24LC64 EEPROM at address 0x50.
The EEPROM contents look like:
[FzgParam] Fahrzeugadresse=1234 MontageplattenNr=1
Fahrzeugadresse seems to be a unique vehicle number, and MontageplattenNr seems to refer to a shared model number - values 1 and 2 have both been observed.
The stock Linux distribution on the device will, at startup, check for a [TestMode] section, and, if present, treat its contents as a shell script to be executed instead of nx. We are not sure what this was used for officially, but it is useful for us to execute our own code without opening the case.