2 Black wires coming from radio

[data89]

This is a 1960's or '70's Polara Model 9300 radio with cassette and graphics equalizer which I'm going to put in my 1969 Chevy C10. The radio part plug has no orange or yellow wires (FYI I know what they're for). However the radio plug has: white, blue, brown, green, red, gray, and TWO BLACK WIRES! I know one is ground (it measures 0 continuity to ground), but what is the other black wire for? It measures 1.5 k ohms to ground.