72 Demon fuse block wire
1972 340 Demon with 4 speed. Wire was obviously cut and new end added. Looks like it might be some kind of ground.Anyway the car ha no exterior lights at all. If you take a test list to the socket it shows power the kicker is the outside of the tail light socket shows power to
Thinking it has to be a ground
There is no ground anywhere attached to a fuse block and NEVER assume that black is ground because it usually "ain't."
You keep saying "exterior lights." Does this include headlights? stoplights? Turn signals? Park tail? Some of these come from different circuits and power sources.
WHAT DOES work in the car.
HEADLIGHTS: Headlights only power comes off the black ammeter wire called the "welded splice" a few inches from the ammeter. This splice breaks off and feeds the power to ignition switch, the hot buss in the fuse panel, the headlights, and depending on year, one or two other things.
Headlights power goes to B1 on the light switch, through the switch, out and down to the dimmer switch where it breaks out to hi and lo, and then feeds through the bulkhead connector.
I would start by unbolting the dimmer switch and see if you have power there with light switch out
Park, tail, and dash dimmer controlled lighting is powered from a fuse which feeds B2 on the light switch, so that is a completely separate source.
Brake lights comes from a fuse, through the brake switch, through the turn signal switch, and off to the rear lights.
It would be rare to have a single point failure causs all lighting to go dark.