The wire was originally a cloth covered wire that loops over the transmission and attaches to the left side pressure switch. the single wire from the same harness that connects the three position solenoid connects to the left switch as well. The switches are wired in parallel so either forward or reverse pressure switch active will provide a ground for the Constant Speed Solenoid (CSS). There should be a bracket that retains the wire on the right and left side of the transmission within 2 or 3 inches of the pressure switch. I have found a good source for cloth covered wire which is Lowbrow Customs in Ohio.
This shows the right side bracket is you scroll down below the cutaway trans pictures.
https://www.sl113.org/wiki/Automatic/StartI am pretty sure I have the brackets that you can have. Unfortunately, I am at sea and away from home for the next month.
I think the full story of the blue wire is this. The wire is crimped into the brackets and it is a very good point for the insulation to fail and short the wire to the bracket. you can't really get the wires out easily so the "Mechanic" unbolted the brackets, pulled the wire out and simply stuffed a new wire over the top with no brackets.
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