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Need Help! Trailer Wiring Gremlins!!!

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So after replacing the 7 pin connector on my 2004 GMC Sierra 2500, I noticed that when I put the left indicator on both lights flashed on the trailer (truck not affected). Every other light works fine on the trailer, including the right indicator. Thought I might have crossed a wire, etc and re-checked the wiring/connections, etc. All fine. Checked the fuses in the dash and under the hood, also all good. So I took a test light, pulled the fuses for the trailer indicators/stop signals on the side of the dash and tested the hot side of each circuit. Both circuits were flashing with the left indicator!!! Got me dumbfounded!?! Not sure if there is another flasher or controller that controls these lights, or what to look for next. Any help appreciated (and needed as I want to tow my trailer to the camo this long weekend!!!).




I should add that the trailer wiring wasn't checked before I replaced the connector as it was too badly corruded to connect to a trailer.

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Nope. Done that already. Put a jumper between truck and trailer and no difference. I've tested it on the truck plug and fuseblock under the dash with a test light on a good ground and get the same result. Is there another ground I should be checking? Where is the flasher and is there a separate one for the trailer? Again, the truck flashers are fine as is the rest of the trailer lights minus the left flasher. Anything else I can check? Getting desperate as the weekend is approaching!!!

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