I backed up my race trailer and my wife noticed the back-up lights weren't on. While checking them at the track last week, the B-U lights were on with the taillights. They're yellow LED's (long story) but we got home before dark.
All I can figure is when I wired the new plug-ins last year, I got a wire wrong. Except all of the other lights are correct and the truck is also correct.
I replaced the rusty GM hitch with a heavier duty one from Curt and needed to change the truck side plug also, that was plug and play but I also swapped the pigtail so that must be where the error occurred.
I bought much prettier Silverado taillights for the 2003 GMC, Dorman sells them with full harnesses so they will be plug and play also, the Chevy side markers alternate with the turn signal.