Gas gauges on A-bodies

the ground for all the other gauges might not be a true ground.
it might just be a point on the board 5 volts less than the supply to them
they think its a ground

but a 12 volt gauge won't

it needs a proper ground

i don't know if that is the case but the idea of a divider in electronics is common

the mid point is used as ground stuff above the mid point sees a positive voltage and stuff below the mid point see's a negative

i.e standard gauges between 6 and 0 ( their ground)
you new gauge in order to work at 12 volts goes between +6 and - 6 (-6 is actually chassis ground)


+6V
i
i
i
i
0v
i
i
i
i
-6V


Dave