My 2002 F250 with 106,000 miles, the turn signals & plate lights stopped working at same time. Fuses and bulbs are good, and have replaced the flasher and directional switch. The hazard lights and others are working. What else could it be???

 

The rear turn signals and the license plate lights are on separate electrical circuits. You either have two separate issues or you missed an observation along the way or one was out previously but you didn't notice it at that time.

The license plate lamps are in parallel with the rear running lamps. Check the individual sockets and wiring at the rear bumper.

The rear turn signals are exactly the same wiring and bulbs as the hazards from the MFS rearward. If the front signals and instrument cluster indicators work, the problem cannot be the flasher unit. Check your brake lights, do they work?

 

It sounds like you have a bad ground. Try connecting a wire directly to the battery - and directly to the outer shell of one of the bulbs that isn't working. If it starts working then you know that that is the problem.

 

A "bad ground" cannot cause his specific symptom. A bad ground at the back would also affect the hazards as well as the running lights since they all share a common ground (G400).

Your "experiment" is invalid for checking grounds anyway, all it's going to do is short out the battery to ground (if the ground is good). If the ground were bad, it would do nothing.