1966 radio feed, part #C6ZB-18A919-A. Called around to a few places, no one seems to have them (or have heard of it). It connects the male/male connector off of the AM/8 to the underdash wiring harness in two locations.
Goes over the the fuse block, radio fuse. This is shown on Jim Osborn drawings for my '65, both in illustration and schematic form. Its simply a long wire with a three female connector on the radio end. You plug in the radio power at that point. Another male wire from the radio connects to the panel light harness.
I'm missing the same harness and have never found it as a reproduction. Geegee is correct. One lead is for the light in the radio and the other is for the radio power. I was able to find the "diamond shaped" connector (one in, three out) under my dash. I guess I'll just have to make my own harness.