My backup lights don't work. The bulbs aren't burnt out. I don't know what fuse it is.

 

Most likely something is out of whack with the trigger in the gearshift.

Hunt around the shifter (auto or manual - doesn't matter) and see if you can find the trigger - sometimes the contacts get bent or dirty, and no longer make a solid connection when you shift to reverse.

What year and trans do you have?

~HH