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The flasher will not be the cause of this as long as the front one is flashing quickly. you need to start at the rear light that is not working and using a 12 volt tester see if there is power and ground to the rear light that is for the turn signal.
Right at the light? and does the power flash using your tester with the signal on?
Can you take the plug out of the back of the light assembly and see what color wire has power?