Without looking at a wiring diagram I would guess that while moving the forty year old wiring around you may have broken the insulation on the ground path wire from the light. Try moving the wires when the light is on to see if it goes out.
Or the opposite, could be on the feed(pos) side if the light grounds at the speedo through the light socket assemble. But I doubt that.
Old wiring so be gentle.
Otherwise one needs a wiring diagram to see what is supposed to turn the light on.
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