Noticed after pulling trailer with a 6 pin connector (trailer has brakes but truck has no trailer brake installed in cab) using a 7 pin (middle is blank) to 6 pin (5 w/middle pin). Left trailer light would not work but truck L signal worked normal. All bulbs f&r are working. I have disconnected trailer connector & checked fuses in driver side door panel. R&R’d L signal fuse. Same. Front headlights changed out to HID w/ halos over 6 years ago & stock rear lights. I have NOT R&R’d the LR light though (yet).
How long has your 2004 GMC Sierra 3500 had this problem?
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