Forget connecting the front and rear lights together. That would turn the front one into a brake light.
Start by checking for 12 volts on the dark green wire at the rear when the brake pedal is being held down or the turn signal is on. For this type of problem, you can use a digital voltmeter, but a test light is more accurate and a much better choice.
My guess is you're going to find the 12 volts missing on that wire. A really good suspect is someone added a trailer wiring harness and used Scotch-Lock connectors to splice the wires. Those don't seal out moisture, then the original wire corrodes over time. In the meantime, I'm working on other parts of the diagram so I can post them later if needed.
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Friday, April 19th, 2019 AT 7:29 PM