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Hi my name is Robert...
IF you are talking about the tow plug that connects to the trailer normally the YELLOW wire is the left turn, if you have a test light you can turn the left turn on the truck side and probe the pins with the test light and the test light should blink with the signal then you would hook the yellow from the trailer side to that pin and the left turn on the trailer side should work. IF you still need more help please tell me how many pins you have on your plug and if the plug is flat or round.... Robert