It is usually a externally mounted sensor, sometimes on the linkage, sometimes attached to the gear box.
For an exact location you will have to consullt the repair manual for your car
reverse light sensor
The reverse light sensor switch on the gearbox needs replacing. The switch is normally located on top of the gearbox & around the middle of underneath the air intake box.
I had the same problem on my diesel 75, it is mounted on the end of the gearbox. I found it easiest to jack up the nsf corner, remove the wheel, then I had good access to pull off the sensor and replace.
R65 Type transmission Disconnect battery. To improve access to switch you could remove the air cleaner. Disconnect the the wiring connector to the switch which is located on the side of the gearbox. Unscrew the switch and remove. Refitting is the reverse of above but a new sealing ring shoukd be fitted. PG 1 Type transmission As above for battery. Jack up car, switch is on top of the gearbox. May need to remove the under shield to get at the switch. trace the wiring back from the switch until you come to a connector and disconnect. Refitting is reverse of above but a new seal should be used.
On the gearbox.
This question omits important required information: Make Model and Year of car Type of gear box.
on top of the gearbox under the clutch lever
this is normally on the gearbox
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.
you can easily locate it on top of the Gearbox Housing...
Could be switch on gearbox, wiring or bulb.
It is important to know the location of the different car parts. The Reverse Gear Light Switch is located on top of the gearbox in most cars.