The tail lights (lit whenever the headlights or parking lights are switched on) are fed from the brown wire. The same wire that is feeding your marker lights (brown) should also tie into the tail lights.
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From Chucks:
This is a typical 1973 - 1987 headlight wiring schematic. Colors may or may not vary from year to year. One of the harnesses I have, had the sockets replaced at one time. The wires coming out of the sockets of the turn signals below the headlights are the same on both sides. Each has a yellow, a dark brown, and a black. Peeling off some tape, revealed which colors they were spliced into. I have a 1977, 1979, and a 1980 harness. All three had the same seven wire colors shown in my diagram. Missing in the diagram is the brown wire that feeds the left front marker lamp. It connects the same as the right side marker lamp brown wire (to the left front marker light).
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