[Richard L]

There is a voltage: need to clarify a bit more please measure these voltages:

Journey start:
1. Ignition key on "off"
2. Ignition key on "accessory"
3. Ignition key on "pre-crank"
4. Ignition key on "run" at idle

End of journey:
5. Ignition key on "accessory" or "stop running"
6. Ignition key on "off"

There is something strange going on. The drawing provided by us is "preliminary" so your feedback is important for us to improve the drawing. Until someone like yourself posting it up, I cannot make alternation.

Basically if you can find a switched 12V that will go to "zero" volt and test pin #5 when you switch off the engine, that will be perfect.