1. Turn the ignition key to the ON position and engage reverse gear. The reversing lights should come on.
2. If the lights do not come on, check the ignition switch, fuses, light bulbs, and wiring.
3. If the lights do not turn off even when the reverse gear is disengaged, check the wiring.
4. If only one side of the light is on, check the light bulb and wiring.
5. To check how the switch itself works, disconnect the wire from the switch, unscrew the switch and connect an ohmmeter to the switch contacts. The switch circuit must be closed when the switch piston is depressed and the circuit must be open when the piston is in the free state.
6. Replace switch if necessary.