Today I finally found out where the ground is supposed to go. Most 70 & later cars have a square welded in nut on the radiator support above the rectangle hole the headlight wiring passes through. The battery ground & headlight ground would both ground to this square nut with a bolt. However very early build cars from the first couple months of production like this one did Not have the square nut welded on. Instead, they used the round hole with a nut & bolt just to the right of where the square nut would be. That is why all the E bodies have a round hole in the radiator support here. It was originally intended for a ground location and then abandoned a couple months into production.
Here is a pic of the correct ground location for such an early car. The car in this pic is an early September 1969 build Challenger.
