look real close at the steering column to main harness connector[s].
i have repaired many over the years due to the terminals becoming loose, resulting in the connectors becoming "melty".
had a buddy with a 72 duster buy a new ignition switch because of this. he installed it, and had the same problem.
as i suspected, that connection was not making a tight contact between the terminals. tightened all but one, which needed replaced [i forget why, but it was an easy replacement], and all has been well these past 5 years or so. he has a spare ignition switch on his shelf.