So what follows is pics of a 1970 Dart. The side marker lights are different so don't let that throw you. There is a wrapped piece of the harness that departs the light harness in front of the radiator support and goes down behind and below the headlight bucket:
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inside the front fender that wrap ends in a 2 wire plug to the side marker light and a single wire bullet connector for the fender top signal light:
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If you look between the wires above you can see the bottom of the fender top signal light coming through the fender. here it is again:
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Here is a better close up of the 70-72 fender top light:
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I hope that this helps you. Since the wires to the side marker and fender top are wrapped there is no way to trace the origin except the diagram on the Factory Service Manual. Good Luck!