Digital Dash finished (and it works!)

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.   

The final result: