Rusty,
that is good news!
That extra wire will not be a ground but it is probably related to the dial light. Originally there were two poles or switches to the volume control. One switched the power to the radio and the other switched the power to the dial light. These switches tended to fail over time and the replacement units only had one pole. Thus they would usually hook both the radio power feed and the dial light together on the new replacement switch. Some repair persons ( politically correct multi gender terms!) would bring the dial light feed out directly without being switched. Your extra wire may be attached to the dial light and the light may be burned out.
Extra Hole: there is no spot for an electrical probe. However, there are various holes in these radios which don't seem to have a purpose.
