It is one of the grounds for the main dash harness, the other is under the light switch. Radios have their own ground wire into the harness grounds. Speakers don't have grounds. This wire is tied to the main grounding wire in the harness along with the other one. Grounds would find the other wire, but I guess if all the lights, radio, etc was on, it needs two wires for the load. Install it. Radio not working is another issue.