Tail lights

Try following the wiring back from the socket to the plastic plugin, and pull apart & clean.

I thought I had lights that didn't work on my 2003, I have a set of lights I never come on. Found out that they don't come on unless someone is in the drivers seat and motor running and shift into gear and then they come on. Sometimes you need two people I have a LT and it is taking me for ever to figure out all the symbols and how to make the settings for me. It is fully loaded and for the first time all the extras work. I also know that GM had a problem with the dash cluster. Had to replace mine, as gauges quit working one at a time. Easy replacement though. I think your truck has about 3 fuse boxes. It you have the manual follow the light schematic, if you don't you can find it on line. And if you have a OBD II run it thru the paces, can give you extra info. Damn near everything now is connected and can affect something else.