[VntgRads]

The black wire in this instance is for radio power.  I work on vintage Mustang radios and the earlier ones use black for power while some of the later 60's units utilize yellow for power.  Looking at the part number on the radio casing (D2TA) your radio decodes as a 1972 truck radio.  There is lots and lots of interchange between these various Ford radios.  I always have to remind my customers that the black wire is not ground but for power!

Craig R