[Hethj7]

I think it is usually black. If not, it should be the largest wire, as it has to handle the most current.

That wire that you believe is some type of negative cable may be the one that runs to the starter....that would be my guess.

Think about how the solenoid works. One post gets power only when you turn your key to start your car. The starter wire is on that post. When you let go of your key, power is taken away from that post and the starter disengages.