140 (early) Dash Illumination
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The early, strip speedometer, 140 & 164 have a rheostat to control the dash illumination when the side lights are switched on.
The coils by now are generally heavily corroded, and don't permit enough voltage to light up the dash.
This is how to work around the problem, allowing a full 12v to the bulbs when the sidelights are switched on.
140 & 164