Why aren't my back up lights working?

I have 2000 Buick century and my back up lights won’‘t work. They stopped working then they fixed themselves and now they aren’t working again. I was hoping to figure out a way to fix it so my boyfriend can fix it so I don’t have to put it in the shop. Hes a mechanic, but he doesn’t have much electric knowledge. So hopefully if I can figure out whats wrong and where everything is we can get it fixed before inspection in Oct. If anyone could help plz let me know. Thank you