Current Draw Down

Good Morning All:
I would appreciate anyone's advice on how to find the source of a current draw down (P6, 3500, automatic, 1972). The initial situation was as follows: When idling with the headlights on, the ammeter would dip severely ( almost to -60 amperes) into negative territory. (I don't know why nothing burned at this draw, but, there was never any smoke or detectable damage.) The engine would stumble and die. Revving the engine would move the ammeter needle to just slightly below "0". This happened consistently. I am confident that the battery and alternator are working fine. Cold voltage across the battery is 12.7 volts. Running voltage across the battery is 13.7 volts. I found a broken terminal on the wire running from the starter relay to the starter solenoid and repaired it. This has helped significantly, but, has not solved the problem. The ammeter now stays at slightly below "0" while running. Revving the engine produces only the slightest positive deflection of the needle. The car, with the highbeam lights on, will now idle without stumbling. But, when I switch the headlights on, the ammeter drops significantly into negative territory (about -20 amperes). I did a parasite drain test and can find no draw down. I consistently get a reading of only 1-2 milliamps. Since the problem occurs only when the headlights are switched on, is it reasonable to assume that the problem is within the headlights' electrical wiring, or, should I look elsewhere. I have had great assistance regarding this problem from two Forum members, and, would appreciate hearing from anyone else. Many thanks!