You may have to "exercise" the hazard flasher button by pushing it in and pulling it back out a few times. Also working the turnsignal switch as well. When you activate the hazards, it shorts outs switch connections in the turnsignal switch and isolates others by mechanical means. If your turnsignals work fine but the rear brake lights "go dead" while the fronts both flash, you may issues with the turnsignal switch assembly which is part of the hazard switch in which you may have to replace the whole assembly.