Yes it could be a short at a light socket.Also a plug in the rear under the bed that splits the sides may somehow have rubbed through and when brake lights are applied has a ground short.I hope this link will send you to the wiring diagram.The diagram shold help.If it does not go though let me know i will send it a different way.
91 ford f150 lighting wiring
It is possible that you have a short in your wiring of course. a few things to check and replace if necessary are the brake switch located under the dash right where the brake pedal is. other culprits might be a failed or failing brake lamp relay. or the turn signal relay. locate the brake relay and put your finger on it to see if it has or is getting extremely hot. if so that should be replaced.
robert
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First the tail lights went out. Could the sockets be wet, or is it possible for them to short out. No trailer lights have been used, no trailer. We were traveling about 60 miles in the rain however when this first happened. The brake lights went out a half an hour after the tail lights.
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