tach trigger on MSD set up

I have an MSD 6a with the tach adaptor 8920 and the MSD website lists 2 ways to hook it up depending on whether the tach is voltage triggered (trigger wire hooked up to neg terminal of the coil) or current triggered (trigger wire hooked up to positive term. on the coil)I had a friend help getting the MSD system installed, so I don't know which tach wire was taken off. I looked at the wire diagram and the light blue wire goes to the + on the tach and the blue/black goes to the "-"

thanks
Gary #06984