Tail lights not working. fuses look good, brake lights work, hazards work, but running lights don't- gmc sierra 2500

Do the front marker/park lights work if not you could have a bad light switch on the tail light side of the switch.you need to check the brown wire at the back of the truck in the wiring harness as this is the tail light wire that feeds the bulbs.if you have power there then you will have to check for power at the socket,if no power at the socket but were you check the wire then there is a problem between the two that you will have to check out.if there is no power when checking the wire then you will have to check for lose of power between the switch and rear wiring.make sure the fuse is good by using a test light and not by looking as sometimes they look good but don`t work.