Where is the fuse for license plate lights located?

If the only lights that are out are the plate lights and the rear parking lights work, it isn't a fuse. They are both powered by the same fuses. All of them share the same feeds. The most likely area for the problem is corrosion in the lamp junction box in the rear of the truck in the left frame rail. The powers and grounds all go there and then out to the various lights, they corrode inside, and you get odd problems. The grounds are shared between the other taillights so it likely isn't those. However, an easy way to test them would be to pull the bulbs out and use a test light connected to power to touch each pin with the lights on. On the ground side (black wire) it should turn on if that is okay, then the feed side (brown) is the problem and it's likely the junction. If you don't want to fight with it and the ground sides test okay, you could just pick the brown lead off the right and the left and connect them to the correct sides using jumper wires to the brown wires at each taillight.