tail lights not working

On my explorer trac, the tail lights do not wok, however, the brake lights,
reverse lights and turn signals work. I've checked and replaced fuses in the
passenger compartment as well as under the hood, replaced the lamp switch,
replaced all bulbs. Nothing has worked. I checked the wires going to the fuse
box and they all look ok. I'm pretty clueless about what else I can do. I have
very little experience with wiring, so any information you can give me would be
appreciated.