Replace the bulb first, if this does not work try the reverse switch.
check nutral safety switch
on what vehicle ? ford Chevy crosley fiat yogo ? there could be more than one switch usually the reverse light switch is located on the trans
check bulbs, fuses & reverse switch
The 2002 Cavalier has a reverse light switch. The switch can be bad, or have a loose connection causing the lights not to work.
Try bypassing the reverse light switch that plugs into the transmission. If your reverse lights come on, voila! Problem solved. Buy a new switch.
check the reverse light switch and all connections
Probably the reverse switch is bad or out of alignment. You'll need a manual to find the reverse switch. It'll be somewhere on the shifter, on the transmission, or possibly somewhere in between.
Reverse light switch bad or out of adjustment.
There is a backup switch in the gearbox. When you shift into reverse, the gear box switch contacts the switch and lights the white reverse lights. If you find that the reverse lights don't work it could be that the switch is not being triggered by the mechanism inside the gearbox. This is providing that everything else is working; wiring,bulbs,etc.
Could be switch on gearbox, wiring or bulb.
The problem is the reverse light switch. It is located on the top of the transmission that should sloved your problem.