No big mystery. The high beam dash bulb is the only one that is not a
diode. Diode bulbs will only light up if the voltage polarity matches
but normal bulbs don't care which side gets voltage. When the ground was
lost for the cluster the voltage in the cluster looked for another path
to take. The bulb for the high beam became that path. The current flowed
backward through the bulb, lighting it up, on the way to a different
ground point in the headlight circuit. Believe it or not that different
ground point was through one of the headlight bulbs. The current was too
low to light that headlight but the electrons passed through it just the
same, lighting the tiny indicator bulb.

Mark

Michael Diehr wrote:
> Success :)   for the archives -- the problem was the T14/3 connector
> was dirty/oxidized.  I discovered this after bench testing the
> instrument cluster and finding it worked properly after making good
> connections to 12V and ground.    Some rubbing with a pencil eraser
> and gentle scraping with a screwdriver on the connector cleaned it up
> enough to work.  I'm still not understanding why a bad ground would
> cause the high beam LED to light, but I guess that's just one of
> life's mysteries.
>
> Thanks again to all who offered suggestion and advice, it helped keep
> my motivation up for solving this gremlin.
>