I'll tell you what it is. I've messed around with enough of these harnesses to know why they do this.
You need to relay the headlights to a direct voltage source from the battery on these cars, because the ammeter in the dash has ALL of the ignition voltage, as well as direct and ignition source accessory voltage running through it.
The ammeter that you have, as well as the red lead that goes up to the flat bulkhead on the steering column need to be checked and either bypassed, or cleaned up.
The ammeter has a peened stud on both ends, that is supposed to be press fit into the piece of metal that crosses over. These studs have the primary wires on an eyelet on the back side, and also hold the gauge into the dash cluster. After the bulkhead at the firewall and the bulkhead at the steering column start to create resistance from old age and corrosion, that red primary wire gets hot, like a toaster oven coil and it starts to fry the insulation board on that ammeter, at the fittings. This makes the ammeter loose and you get a ton of voltage drop across evertything.
The best thing to do is drop the steering column, pull the gauge cluster out of the dash and remove the ammeter, so you can set the studs to the metal with solder. Even if you get a replacement that is tight, it's still 40 years old and still a press/ peened fit.
I would also recommend removing the red primary wire, and maybe the blue and yellow ones from the steering column bulkhead and bypass that bulkhead with some bullet connectors. You may also want to check the large primary wires at the firewall bulkhead to clean those up as well. Start at the ammeter. Nothing at all that car will work with it disconnected, so its critical to have a sound connection, there. It feeds everything and the entire system, including your coil feed, bottleneck into that single junction. An ohmmeter or test light will tell you that its good, until you check voltage across each post, when its been running, then you will see the voltage loss. Just pull it out and bridge it really good with solder and flux.
The idea behind running a new primary lead off of an auxilary battery positive feed, to a relay that will run the headlights and use the existing feed wires to trigger the relay, is to take that voltage draw off of the ammeter. You won't see any more headlight dimming issues or weak ignition problems at night, if you at least take care of the bad ammeter connections and you won't see it flinch at all if you relay the headlights.
