My reversing lights aren't working (Disco 2). I've replaced the bulbs with no joy. I've looked in my Haynes manual and fuse 25 is responsible for these and the brake lights. The brake lights work though so what could it be?
If it's a manual gearbox Disco, it'll almost certainly be the gearbox switch. Accessible via the LHS (nearside of the gearbox) old one screws out,new one screws in - literally a 2 mins job.