I think you just found your problem......good work! Since the two battery strings terminate onto the Trombetta (isolation/interconnect relay) just make sure to disconnect the two battery strings (common chassis ground cable off the battery post on each string works if you don't have individual disconnects) to prevent any welding/sparks if either relay cable end touches chassis or some other unintentional connections. You can also use your voltmeter to ensure both Trombetta cable ends are dead before starting to disconnect them for the repair. Many folks also slap a wrench onto the battery nuts and start swinging it without realizing that wrench can become a dangerous conductor if it hits the other battery post etc.
It is also a good habit when sticking arms and hands into battery boxes to remove metal watches and rings......it is amazing how fast those items will vaporize along with potential burns to you from the heat generated. The battery voltage is not dangerous, but the current can be. You will be fine just take your time and work safely. End of Motherly advice.....I have seen it happen is all.
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