I am working on what is supposed to be a 57 GMC radio right now. There are three wires coming out of the main chassis. One is dark or black and it has a fuse connector on the end. This is the main power wire. The chassis serves as the other ground power connectio.
I also have a yellow wire which has a spade connector. This is the wire powers the dial light. The third wire is green and it has a female connector on the end. This is the speaker connector.