Shot in the dark
Glancing at the PlanePower website, it appears that at least some of their regulators illuminate the alternator fault lamp by pulling the lamp's "negative side" to ground. So if the lamp's "negative side" wire is making contact with ground some other way, the lamp will turn on.
Another possibility is that the lamp is controlled by a Normally Closed relay that has to be energized to open the circuit to the lamp. In this case, a bad connection on the control-side of the relay would leave it closed (lamp illuminated).
Bo
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