This one, the wire corroded through right where it connected inside a plastic plug cover, the only place where the wire was bare - quite hidden from view. Probably seen it before and knew to check there.
You figure the intermittent performance was that wire was just close enough for the electricity to cross the gap, and as soon as we tried to move the car it would short dead. A skilled tech would know what that means, but only a very close inspection could have found what they did.
I teach computer diagnostics - and in my professional opinion that was some grade A+ electrical system auto diagnostics in action. They know their stuff.
I'd put that mechanic in retainer. Glad you're up and running.
Connections under the plastic cap would have to be bare for proper contact whether positive, negative or ground. A loose/corroded ground wire can cause the problem you had and mechanics will look for that next after the obvious battery, alternator, starter checks. Coating the connections with Penetrox, an anti-corrosive zinc paste will help.
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