License plate lights are in the tailgate and those wires run thru the hinges and then into the tailgate body...check for broken wires in the harness at the hinge...don't remember which hinge side (R or L) for those lights. Open the tailgate and you can see the wires where they cross from the body to the gate.

The other tail lights...the wires are inside the body ...a simple test light will tell you if you are getting current thru the wire to the bulb holder...