You're gonna have to follow the wires back as far as you can. I know it sounds like a PITA but if it's showing 8 volts to ground and no ground connection on the other plug, something is feeding it and it's not direct battery voltage. Get me a shot from the front of the plug so I have an idea what we are working with. Take pictures along the way and report back with the results. Only way anyone here can help you identify what it is and where it's supposed to be connected.