If the fuse is OK....is it OK???
Next you should lift the Mog securely and get your head under the gearbox. On one side will be a plug with two wires. If you're really unlucky it will be on the top.......
One wire (the feed) will be live when the ignition is on. Pull each wire off in turn and test with either a multimeter, or test lamp. Wrap a small piece of red insulating tape round the live wire for future reference. If neither is live go back to step 1. If the fuse is OK, there's a break in the circuit - probably a corroded connector.
Next select reverse gear and test that the other wire on that switch becomes live. If it doesn't - jiggle the switch about. If you can get it out (unscrew) you may be able to dismantle and clean the contacts. Once the second wire is energised by that switch, test the power at the lamp. If no power look for dodgy connectors. If power - look for a dodgy bulb holder, or dodgy earth.
