Alternator meter is jumping around and the lights are dimmed... won't stay running

MAYBE you have some short going on instead of an open. Try figuring out how to isolate "stuff" from the problem

EG you can pull the wiring loose supplying the car interior as well as the alternator. Start with the starter relay. Remove the wiring from the "big stud" on the relay and follow the one going from there to the fuse link to the bulkhead connector. Leave that off, put any others back. Now check for power at the altenator stud. Should be NONE. Check for headlights. Should NOT work.

Now to run car, remove the "key side" wire from the ballast and jumper power to that. To start car, jumper across the two exposed terminals of the starter relay. This should get you "starter" and "ignition" to run the engine and nothing else. Will it run? Is it stable, act "right?" If not fix it.

Then reconnect wiring and figure out.........are you "losing" a feed connection (open/ poor connection) or do you have a short happening? (pulling down voltage). Is there wire smoke or smell? Hot plastic?