Positive ignition lead (back of truck ideally)

[BCRanger]

Hey all, I'm installing a DC-DC system and because our trucks have a smart alternator I need a positive lead that gets power on ignition (not accessory) to clue in my charging system that it is ok to pull from alternator even though voltage is lower than expected. Any ideas if there is a circuit I could tap from back for that? If not, what would be my best bet in engine bay?

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