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A 2nd alternator and completely separate 12 volt battery system independent of your F-150s is another option. The Pro Power 2.0 kW system includes a 2nd 24 volt alternator installed under the hood, my truck has it. You could maybe use that location for the suggested 2nd alternator for your separate audio 12 volt system.
As someone who bought a lot of car stereo equipment back in the day, I'd say this solution will be by far easier than modifying the existing system (along with boosting the idle speed via Forscan). And will prevent any unintended issues from trying to modify the existing system. Its the only way I'd go, unless my truck had the 7.2 kw AC system and then I'd tap that instead.