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The factory radio grounds through its own chassis. The pin that's sometimes populated with a black wire on aftermarket adapter harnesses is connected to nothing on the car side. Empty box on the factory wiring diagram.
IIRC my old '90s Pioneer head unit, while the manual directs you to ground through the black wire on the head unit's harness, is also simply grounded through the brackets to the metal frame of the radio.
Basically everything else in the interior (cluster, accessories, everything) grounds to M57 which is up under the passenger side dash.
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