Suddenly Lost Power

The pigtail is there to connect to an existing body ground if so equipped (most later cars had such a thing) Easy enough to add, and a pretty good idea. Get some black 10 AWG wire and a ring terminal large enough for a fender bolt (3/8). For 10 AWG you need the yellow ferrules. Again, you want to clean the metal under the bolt and/or use an external tooth lockwasher between the terminal and fender. If you dont mind drilling a hole in your inner fender, you can just get a self tapping sheetmetal screw and the apropriate sized terminal ring. Better yet, if you're gonna drill a hole for a new screw, attach the wire to the radiator support instead. Thats where the headlights ground and they are your largest electrical load on the body.

If your car sat for a long time or was in a corrosion prone environment, you may want to consider cleaning and inspecting every electrical connection on the vehicle. Kind of a pain, but potentially worth it.