I'd say that if you can duplicate the problem by wiggling the harness you may have narrowed down the source of the issue.
Harness issues do NOT fall into a "common failure" category, with the exception of the "pigtails" to the coil or alternator, so I can't confirm this as a likely problem.
Testing it carefully to confirm it's an internal harness issue rather than the harness pulling on a connector with poor contact would be the next step. Only then would you want to "go inside" the harness cover to continue trying to narrow it down.
A number of members have 'peeled back" the harness cover & rewired/rearranged things to create a "cleaner" appearance, so it CAN be done. You'd want to open it up & continue the testing to see which wire or wires is causing your problem. IF you don't get lucky & find a "new" junkyard harness cheap to just swap in.
Most wiring issues happen at or near connectors, that's why I mention checking those areas first B4 diving too deep into the harness itself.