I had the same problem with my '97 DeVille a couple of years ago. The instructions that I was provided (which worked well) are as follows:
(1) Remove license plate
(2) Remove rear compartment lid trim cover (inside trunk). Not necessary to remove whole cover, just the part that conceals the license plate pocket.
(3) Remove license plate pocket by removing two nuts and bolts that hold it in place from inside the trunk.
(4) Remove the license plate pocket from the trunk lid, and this will give you access to the bulb.
(5) Replace bulb and reverse removal procedure to put everything back together again. You will probably need to purchase replacement black plugs to refasten the lid trim cover, as these aren't generally reusable once you pry them out of place.