There are 2 grounds involved: each tail light has a separate ground that may need to be unbolted, wire brushed & reattached. But more likely is the central ground point under the dash, behind the gauge panel on the console. Above the tach is a stud to which is connected maybe 6 or 8 separate ground wires. Condensate or a leaky windshield causes corrosion. Unbolt the ground, wire brush all the connections clean and re-tighten. Loose or corroded wires here causes weird and unpredictable electrical problems. Good luck-
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