I didnt own a 1993 Park Avenue, but I did have a Buick Lesabre from the same year, and I know they were similar cars. The Lesabre had something like 15 or 20 screws to remove to get to the radio. Its been a few years but heres what I remember,
Remove that long strip of fake woodgrain, should pop off.
You should see most of the screws you need to take off. There are more under the edge of the bottom of the dash, (3) in the Glove box, and (1) in each AC vent, (4 total). There are also (3) under the flap that overhangs the instrument cluster.
Thats all I remember off the top of my head, I hope that helps some. I hated taking the HU out of that car lol, took me forever to find the hidden screws in the AC vents.