My day time running lights won't work but my headlights and taillights come on with my light switch off. This is on 99 chevy Silverado

 

My 1998 K-1500 doesn't have DRL's. Instead, the headlights come on at reduced power. I really like that function since I use lights on day or night. If need be I can pull the fuse, or easier, just click the parking brake foot lever down on notch and the headlight are off.

 

Check your DRL light bulbs and sockets. When I had my 99 Silverado the DRL light bulbs would burn out every so often. There was also so much heat there that the sockets occasionally melted some causing a problem.