Tag light fuse location

There is no separate fuses for those lights, they are powered from the same feeds as the marker and parking lights, the Left and Right rear park fuses. Common failure points for them are the lighting junction box under the rear of the truck and the ground wire at the bulb socket so you end up replacing the socket itself. Are both out or just one? All of the other marker and parking lights on the rear are okay? Does your truck have the rear steer option?
If you remove the lights they will have a Brown wire and a black wire. The black is ground the brown is power from the other light. If you take a test lamp and connect it to ground, it should light up when touched to the brown wire connection in the socket. If it doesn't but the rear tail lights work then it is likely a bad connection at the wiring junction box. If that is the case you have a few options. The box simply connects the lights together inside it and then sends power out to the plate lamps. The easy repair for that is to simply connect the Brown wire from the tail light to the same color on the plate light. The harder repair is to replace the junction box, they get corroded and cause problems. It's mounted on the rear on the frame cross member under the box. It uses simple plug in connections. As the first test you might just want to unplug the one that feeds those lights and see if it is corroded or damaged. Then go from there.