Ok, since there is nothing on the forum about how to install an aftermarket stereo in a 2000-2001 catera, I'm gonna share my experience. Unless theres a magical kit that fits perfect, you'll have to modify it to make it work. The install kit I bought was by Best Kits. It says its for a 96-01 Catera/ Deville/Grand Prix. This is the brand a lot of installers sell. I bought it from a professional shop.
As with everything else on this car, it did not install easily. I had to modify the pocket to fit in the opening. The pocket was 1/2 inch too wide. I had to cut 1/4 inch from each side of the pocket all the way up to the shorter of the 2 mounting "legs". Do not cut all the way to the front opening of the pocket! Stop about 1 inch away. I also had to trim a couple other spots on the kit to get it to fit.
I removed the mounting bracket from the car to make it easier to measure everything. You can see here how much wider the pocket is than the opening it needs to go into.
Here's the kit after I cut the sides enough to fit. Theres also a whole cut for my iPod cable so it can come through the pocket.
Heres another view
The shorter leg is about 1/4-1/5 inch too short. I just skipped putting screw in them. They are the bottom legs. You could use some washers and a longer screw I suppose.
This tab has to be removed from each side as well.
I didnt close in the sides of the pocket because the mounting bracket is black plastic too so it looks fine.
And the final product
This was the most modifying I've ever had to do on a stereo install. If you measure carefully and keep your cuts clean it will turn out nice. Remember to but an equal amount from each side of the pocket or it will be off center.
As for the wiring harness, it was also a Best Kits brand. The only thing I had to do was attach the red ignition wire to another ignition switched wire. For some reason when I plugged it in, there was no power to the red wire. I tapped into the plug that houses the heated seats (Which I dont have). Otherwise it works fine. Hope this helps. I know I sure struggled with finding any info on it. Good luck!