Factory tach wiring??
This is a tough one. The '70 manuals seem to have ignored it. I found a drawing of the rear of a 70 Dart that indicates the bottom terminal originally had a flat spade hooked to it. Looks like the top terminal is switched ignition, and the bottom goes to the coil neg. In the original drawing, they are both incorrectly labled "dark blue" which only applies to the top one I believe. If you are running aftermarket igntiion like MSD, refer to your ignition system instructions, it may be different.
Drawing of rear is '70 Dart:
This is a tough one. The '70 manuals seem to have ignored it. I found a drawing of the rear of a 70 Dart that indicates the bottom terminal originally had a flat spade hooked to it. Looks like the top terminal is switched ignition, and the bottom goes to the coil neg. In the original drawing, they are both incorrectly labled "dark blue" which only applies to the top one I believe. If you are running aftermarket igntiion like MSD, refer to your ignition system instructions, it may be different.
Drawing of rear is '70 Dart:
