there is an open somewhere inbetween the "ST" (start) terminal on the ign switch out to/thru the bulkhead & out in the yellow wire to the starter relay "ign" terminal. Start at the ign sw (or the bulkhead) & see if the specific bulkhead terminal in the firewall half is hot while cranking & work upstream or downstream from there. unless there has been too much amperage (somewhat rare) that has burned up the strands or all broken strands from too much flexing (extremely rare), wiring goes bad at the terminals/connections.