The final wiring diagram:
Note the relay for the 12V dimming. The reason I installed it is that when you apply the brakes and have the turn signal on the 12V Dimming input generated an intermittent ~0.8V to where the dash thinks it needs to dim, and so it did. Bright-dimmed - bright - dimmed at the pace of the turn signal. We tried to suppress it with pull down resistors without success so I resorted to installing the relay as a gate keeper to not let any voltage through.
The dash loads like you would expect and all readings, including the now correctly scaled fuel gauge, are reading correctly.
