Low voltage = no charging

Explain what this blue/ yellow wire is that you are unplugging?

The "high" (battery) sided of your ballast, one of the field connections, and the IGN terminal of the regulator (blue) are all supplied from the "ignition run" contact in the igniton switch. First thing I'd do is make sure all three of those are "same as battery" or within .2V (two tenths) or so. More indicates a too much voltage drop. Your path is from the battery, to the starter relay stud, through the fuse link, through the bulkhead, to the ammeter, to the ign switch connector, through the switch, back through the bulkhead connector, and to the three feedpoints I mentioned above.

If these are OK, do a "quick check" on your alternator. Confirm that you have battery/ key on at the blue field connector. Unhook the GREEN connector, and run a clip lead to ground, from the exposed alternator field terminal with with the key on

You should see a spark as you hook/ unhook the clip lead.

Now start the car and gently give it some RPM while watching the ammeter/ voltmeter. The voltage / amperage should come up, you are operating the alternator at what is commonly called "full fielding."

If nothing happens, you probably have something wrong in the alternator. Make sure you have battery at the output stud, and check as above while running to see if you have (high) voltage at the output stud. If not, alternator problems.

If it DOES charge with the field connected, double check continuity of the GREEN wire back to the regulator.

With engine OFF, but key ON (run) check voltage at both connections inside the regulator plug. You should have "battery" at both. If you do, probably a bad regulator.