Hey! What's up!
2001 Buick Lesabre Limited here.
I recently added an aftermarket HU. It was a pain at first but now it's a peice of cake. Here's how I did it.
First the lower part of the dash on the drivers side pulls off, then on the pass side open the door and the dash end snaps mostly off. Open the glove box (there's a screw in there), then the wood grain across the dash snaps off. and look for several screws across the lower dash,and one under and back that needs a long screw driver, about at the drivers knee.
Hope that helps.