From a previous post:
Both tail light bulbs. They still work though. Even with them out the directionals don’t work.
Removing lamps/current load from a circuit using a thermal type flasher will not improve performance - if anything, doing that will cause the flash rate to slow down. If enough lamps are removed, eventually the flasher will cease to flash altogether.
From a previous post:
The reason that the front directional lamps illuminate - but do not flash - is most likely due to a reduced current flow through the flasher relay. Flashers require a certain amount of current flowing through them to cause the internal contacts to open/close.
Maybe check for voltage on the DK GRN & YEL leads where they enter the harness connector. Also be sure you rear harness lighting is properly grounded. With an 1157 lamp (i.e. dual filament), the brake/blinker filament draws a lot more current than the filament for the parking light. The filaments are rated at 30/4 W and the amperage flow would be proportional. The side marker lamps are rated even lower - like 3W. Circuits with higher current flow require a better path to ground - than circuits carrying low amperage.
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The rear lighting harness - in the images above - is wired with two grounds. Both lighting the L& R assemblies has it's own dedicated ground lead:
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That design often helps in diagnosing a problem; If the lamps in one assembly are out - but the other side is fine - it generally points to a loss of continuity in the ground leg of the affected assembly.
But don't assume you have this type of harness. There is another design that was widely used. In that type, the ground legs from both sides are led to the middle of the truck and spliced together. Then that common ground wire is bolted to the frame. So, as you can imagine, if that common wire breaks or the connection to the frame is lost...both sides go out.
But, even having said all that, I'd bet my left nut that when you break the connection at the plug and check for voltage on the DK GRN & YEL leads - you get nothing. I would use brake light circuit to generate the test voltage. It is a steady voltage (no flasher to confuse things), it only goes to the rear lighting and it is fed directly from the battery and not though the ignition switch. Jam a cinder block or something similar against the brake pedal. While you're doing that, take a second to confirm that voltage is at least coming out of the brake switch (on the white lead):
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