nancy , Do you have rear signal lights? If bulbs, bulb sockets, fuses and brake light switch all appear to be good the fault is probably the turn signal switch. Also there is a trailer light harness on the vehicle check at that connection or actually remove the trailer light harness as some have a built in converter that can cause weird issues also. Links below may have some useful info. for you as well. Good luck.
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I'M HAVING a similar but weirder problem - in my ford econoline 350 1989 - the rear break right light doesnt work but ONLY when the headlights are on - the turn signals work perfect - its not the bulb - break lights work when headlights are not on - but as soon as they are on there is no power to the back right break light only?? please help
von Mark daniel Abadi
Mark, I had 3 different 89 econolines, all with that issue. its a weak ground. what you need to do is find which wire is the ground wire coming from the light, and splice an additional wire onto it, and attach the other end of the new wire to the body or frame.