Hy lowbow, as waldo53, has said if the voltage regulator BAT terminal is not hot the way the harness is attached, then you will need to make a connection between the light switch and the ammeter, or the BAT terminal and the ammeter. The way you have described how the harness is connected, I would be suprised if the regulator BAT terminal was hot, and if it is not, you should call the harness manufacturer, and tell them of the error in their design, hope that helps.
