joatmon wrote:Is the missing black part still stuck in the back of the radio?
No, and I checked all around underneath and behind the dash in case it was dropped. The previous owner had installed an aftermarket stereo at one point so I'm wondering if it was accidentally pulled off and lost. The wire as-is doesn't fit into the stereo -- it's basically like connecting two male ends together.
I haven't had any success finding that specific end piece online anywhere.