There are three extra ports in the fuse panel that GM put there for adding accessories without tapping into existing circuits. They look like half a fuse slot (just one flat blade connector instead of two) and are labeled BATT (constant power), IGN (ignition switched power), and ACCY (retained accessory power - like the radio).
White 2000 Trans Am (sold)
White 2011 Mustang Convertible
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