Classic/Antique Car Repair: electrical problem 1975 Plymouth Scamp, plymouth scamp, slant 6

Question
My issue is that I have a 1975 Plymouth Scamp with a slant 6.  Recently when the headlights are on and I turn on another item I notice for a split second the dimming of the headlights.  Then when I turn on the heater/blower at idle speed or low speed driving, with the headlights on that the vehicle will shut off.  Has brand new battery, new regulator and the alternator was benched tested OK at local parts store.  I am looking for any help or ideas on where to go next in my search. ANy help will be greatly appreciated.  Thank you in advance.

Answer
In that era Chrysler built cars had a lot of problems with the main wiring harness connector that ran through the fire wall just to the outside of the steering column. That would be the first place that I would look for corroded and burned connections. Are you handy with a voltmeter? If so check the charging voltage at the battery with the engine running both without the lights on and with the lights on. That voltage should be above 13 volts. Let me know.
Brad