If the dash lights are working...... The fuse box is prob connected. Get a test light and test the brown and yellow wires at the tail lights.
If they have power...... it's a bad or corroded socket or the ground is bad. I'd clean the ground and sockets 1st and retest.
If they don't have power........ Find the brown and yellow wires under the dash, I'm not sure, but they should be run behind the driver's side kick panel. follow them all the way to the light switch, and brake light switch, looking for any breaks etc........ then using the test light, check the output from the light switch and brakelight switch. if you have power out of the switches...... you have a break in the wires leading to the rear of the car.
If they don't have power...... Use your test light and check the fuses. If they have output it's either the feed wires from the fusebox.
If they don't have power...... disconnect the battery and pull the fusebox apart and make sure the terminals are still connected on the back side of the box
I don't think it's the fusebox, because your dash lights work.