Hi Graham et al
Was busy with parties and work...
Yes I believe there is more to the LED circuit in the dashboard than diagram shows.
Probably something that requires specific voltage on the LED signal wire.
So if ECU short it or its open circuit then its lit.
This can be seen on some diagrams of other cars (Google).
I have a friend which has good electronic knowledge - we will try to work something out.
In the meantime I'm going to try a used unit from BBR!
I'll be back after playing around...