'66 Valiant Weirdness

Show us the regulator. The original one was a mechanical like a relay. Modern replacements should be solid state. IF your new one is solid state, this is likely the age old voltage drop/ "ground loop" problem.

It might be that whatever ignition you installed draws MORE CURRENT, and it is powered off the same circuit as the VR

The circuit path "in general" for the ignition/ VR power for these girls, in general is...........

Battery-----starter relay stud-----fuse link----through the bulkhead connector on ammeter RED wire----to and through the ammeter-----out on the ammeter BLACK wire----to the under dash fuse link----------to the igntiion switch connector---through and back out the switch----back out the switch connector --(on dark blue "ignition run" or IGN1 wire----back out into engine bay through bulkhead connector-----to underhood ignition loads, including VR and ignition system.

THAT CIRCUIT by the way IS NOT FUSED so be some careful

The most common cause of problems is VOLTAGE DROP in this circuit.

1...Turn key to "run" engine off. Check voltage with meter between VR IGN terminal and battery POS terminal. Do not disconnect any wireing, do this with everything connected normally. You are hoping for a very low reading, the lower the better. Anything more than about .3V (3/10 of one volt) you need to look into

2....Ground circuit. VR MUST be at "same as" battery NEG. Start and run car, let battery become "normalized" and with engine running on fast idle, do this test first with everything "off" and again with heater lights, etc powered on

Stab voltmeter probe into mounting flange of VR and remaining probe into top of battery NEG post. As before, you are hoping for a very low reading, zero is perfect. Any voltage a tenth or two, improve the grounding between battery and body/ VR case

VR MUST be scraped clean and firewall, mount with star lock washers.