Power to everything Nothing works

Sounds like maybe ground to me. Try testing UNDER LOAD. AKA you put your light/ meter on some point, the battery + terminal, the starter "big stud," or the starter either post and turn the key. Does it still light? I bet not.

YOU CAN NOT TELL by looking. I myself was fooled on my 86 Ranger years ago. The ground cable went from the batt neg to a factory "T TAP" at the frame. That was good to the frame, the lights worked GREAT. But where that T tap was crimped, up in the cable going from there to the engine block, it was severely corroded--no crank.