Not sure about a 69 but on a 70 the main chassis ground is from a pigtail off the negative cable at the battery to the passenger side front fender. There are also several grounds from the gauge cluster to the metal frame inside the dash and one to the emergency brake bracket.
As you know the headlights are grounded to the core support. Not sure about the tail lights on a 69. You can disconnect one headlight or tail light at a time to see if it's one that has an issue.
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Hank
70 Chevelle SS396(454), M20, 3.73
My Chevelle
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