1998 Mustang Cobra, back up lights not working

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I am not sure how long the back up lights have not worked (girlfriends car). The 15 amp fuse under the dash board was bad. Although the new fuse has not burned out since replacing it, the back up lamps still do not work. With the ignition key on and the (5 speed) transmission in reverse, there is no voltage to either lamp, and no voltage to the transmission reverse switch. The wiring harness does not appear to be damaged. It is not clear how the wire is routed from the fuse box to the transmission (without dismantling the car) BTW: The hazard lights do work (according to the manual, they are on the same circuit)