Could be a fuse or you need to make sure you gear lever is in the right place as it's used to activate the reverse light switch. If the gear knob and stick rotate, the grub screw that locates the gear stick to the top of the gear box will need doing up.
Check your brake lights are working as well as they share the same fuse. I had an issue with the reverse switch shorting, which kept blowing the fuse. Not an uncommon problem by all accounts - check gvarleys sagaris page.
