Reverse lights not working

I don't have a good electrical diagram for this vehicle but I think that the reverse lights are controlled by the transmission range switch on top of the the transmission where the cable hooks up. Sorry I don't have more info for you. Yopu may want to go to an auto electrical shop or a GM specialist. If you want to find a specialist, here's a directory link for you: https://repairpal.com/buick-repair-in-fort-polk-louisiana