First, Welcome to the Club
I have done this. I installed one in my '02 SE a while back and it most definitely is possible. There are however a couple of requirements.
1. All 2000+ radios in the Bonnevilles use the class 2 serial data bus. This is only part of the requirement. Your factory head unit MUST have a 'BAND' button. If you only have "AM / FM" you are not satellite radio ready and would have to get a more current HU.
2. You will not have the wiring or connector required to plug directly into the car. The wiring can be added. I'm not sure what your technical skills are, but there are a total of seven connections which need to be made to facilitate installation. If you have a 12 disc changer in the trunk the process is simplified.
3. If you don't have a 12 disc changer, you MAY not have the wiring going to the radio connector. Without this wiring it becomes more of a challenge because you need the ability to add three terminals to the Dock-N-Lock connector in the back of the radio bay. The terminals are virtually impossible to find. If you decide to try this, I can guide you through the process to determine what wiring is in place and what you will need.
Here'* the down side... I found the Sirius SIR-GM1 tuner to be VERY slow in responding to the operator changing the tuning **** when compared with factory XM. Also, the display on the radio would periodically hang and wouldn't update unless I changed stations. There were instances where the satellite receiver would not process the command on the class 2 data bus to stop sending audio and I would hear both the 12 disc changer and satellite over top of each other. I would have to remove the fuse for the Sirius box once in a while to correct this. I don't believe this was an isolated defect in the tuner as it did this with two different, brand new products.
The seven connections required are:
1. +12 VDC
3. Audio Common
4. Audio Left
5. Audio Right
6. Audio cable shield drain
7. Class 2 serial data bus.
Please post whether you have a 12 disc changer or not. This will help me to give you more information.