The reverse light switch is screwed into the left side of the gearbox housing behind the front wheel drive shaft pivot. Before removing the switch for replacement, check the integrity of the F3 fuse on the block in the car.
You can check the operation of the switch using the tester in the mode «dialing», for which disconnect the contact connector from the switch and check, by setting the reverse gear, that the switch is working.
If the switch is defective, unscrew it and replace it with a good one by installing a new sealing washer. If part of the oil leaked out of the box during the replacement, bring its level to normal.