Go hunting for them, there are plenty. Almost all earths are white with a black trace (Apart from some of the thicker ones which go between mechanicals and body, which are black with a yellow trace). If its either of those with eyelet (the round hole type connection) and its disconnected, bolt it down to something metal.
That one on the glove box just sounds like the one for the factory radio, which I'm guessing you're not using, so it doesn't matter.
If you're still having problems, take it to somebody who knows what they are doing, and owns a multimeter.