Are you sure you have all the original ignition system (distributor etc.) still installed?
If it has been modified/upgraded in any way that wire may not run directly to the coil anymore e.g. I rerouted mine to the MSD 6A box when I upgraded to electronic ignition. There is also a common relay upgrade to take the full load off the ignition switch (the main cause of failure of the igntiion switch electronic internals) that would have a low current circuit switching the full load.
Maybe you'll have to reverse trace the wire from the opposite end i.e. from the ignition switch.
Julian
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