If the black wire is on the + side of the coil, which is the 12 volt power to the coil, it is likely to be the 12v feed from the immobiliser. From the age of you +8 I guess it will have an immobiliser. For +8 without an immobiliser the power is via a white wire which may have a green tracer.
(My experience is that often on security modules all the wires are black to make life difficult)
There is, or should be, another wire from that same terminal on the coil which is white and goes to provide 12v power to the ignition module
Mike
