The Stovebolt Site Forums The Shop Area The Electrical Bay The Radio Bench Radio harness has 2 black wires

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, ( I know what they're for) 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.