I have different lengths of wire, with a clip on the ends. the wire with clips on it's ends, I would attach to the connector, and look for a ground source, to touch to or attach the other end to, I think you are right to think the second wire is the ground. If power is on to the running lights
, and it can be determined where other items ground to, ie. metal surfaces or a bolt to the frame, or look for white or green wires and what they are attached to. Is the first wire hooked into another wire? There should be a couple of wires under the license
, going to the stop, tail light
as you look into the compartment, the tail light is on the corner with wires coming out going down to the floor, it will have a ground wire you can attach to.
hope this can help
P.S. you should get a test kit to check hot and cold wires