I have a 2005 GMC Sierra, but it should be identical for this reason.
There is a light sensor on the dash near the windshield just right of
center. Its purpose is to turn the headlights on & the DRLs off when
it gets dark. Perhaps the sensor or the wiring has failed. The
sensor might also be dirty. Try cleaning it.
I also found one time one of my kids had placed a kids book on the
dash blocking the sensor. This kept the headlights on until I
realized what happened.
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