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Q: 1998 Mustang Cobra, back up lights not working on 1998 Ford Mustang

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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)
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It sounds like you have approached and diagnosed this problem very accurately, nice job. If you have power at the fuse and not at the switch, then that wire either is broken, or has a bad connector.
If you look at the diagram, between the fuse and the switch there are a few connectors. Starting from the fuse, you will need to locate each one and check voltage in and out of the connector. Follow the wire until you find the problem.
Let me know what you find!
Found the problem. The wires to the connector at the reverse sensor switch did not have any slack. The wire from the fuse box had separated from the connector, only the insulation remained attached. This was apparently caused by movement between the body and transmission (aka: torque induced by a lead foot)

After repairing the connector, the backup lights still do not work. The reverse sensor appears to be bad (no continuity when transmission is in reverse)

The good news is that when the wires at the connector are shorted together, the backup lights work.
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