try what dave and patrick suggested.if that doesn't work check your turn signal assembly in the steering colomn.i just had a problem with mine.after getting it back from being painted,everything worked except the rear left signal and the rear left brake.i checked ground,bulbs,wiring,connectors you name it.i didn't think it was the signal switch because when i tested it with volt tester it showed two hots comming from the switch.as i later found out there should be three hots comming from the switch when you step on the brake.if you can find a new one ask if they will let you see if it is the right one.once you get it home unplug your old switch and plug the new one in.you don't have to remove the old one from the colom.when you plug the new one in try the signals and the brake lights.if they work it is probaby your switch.
good luck:elc66