check the reversing switch ,
remove the jack plug from the reversing switch, bridge out the wires on the loom connection lights will come on permanent does the fuse BLOW ? if NOT change the reversing switch. if YES problem could be the reversing lights have corrosion internally in the units, or the wires leading up to them have corrosion, check the wires for chaffing where they pass through holes in the car body.
wirecutterman
November 2009