Backup (reverse) bulbs on 91 Buick Regal

I have a 1991 Buick Regal. Neither of the backup (reverse) bulbs work. How do you replace them? The lights are integrated into the rear bumper, and the bumper appears solid from the bottom. How do I get at these? (I did a quick search online, and found nothing. There is also nothing in the Owners’ Manual regarding this.)



Any help would be appreciated! Thanks!

Remove the trunk panels if they have obvious fasteners and you might find the sockets.