I just did an install of the SIR-GM1 unit that interfaces with the stock XM ready radios in the 2001+ GM models. The harnesses that come with the unit only works if you had XM installed from the factory so you will need to make some modifications if you didn't have XM.
I did this install on the non-Bose unit. I'm sure you can do it on the Bose or the Nav system, but I do not know the pin configurations.
SIR-GM1 Pin configuration 
Seville CD Changer Pin Configuration (we connect through here)
When connecting the SIR-GM1 unit to the car, you just have to match the pins. Because the SIR-GM1 does not come with a useful harness, you need to make your own. Take the supplied harness (the one used to connect to XM installs) and cut all the cables away from the plastic connector on the other end. Use a soldering iron to add some connectors to the ends of each wire (use some shrink wrap on each one to keep the wires from making accidental contact) It will be a lot cleaner if you use shrink wrap on the wire group but this isn't necessary.
My Harness (I thought that I could install the SIR-GM1 behind the glovebox so I made a longer harness. It turns out there there isn't any room so you won't need to make it longer. Your harness will only be about 7 - 8 inches)
Now lets connect it to the car. The connection point is in the center console between the front seats. If you have a CD changer you can still do it but I don't know how you would need to alter the connections.
The two bucket trays in the center console lift out and just below them, there is a little secret compartment that lifts out as well.
Once everything is out, you need to remove the little trap door that is located down there so that the unit can sit flush on the bottom of the console. I used some thick double sided tape to get it to stay put.
Bend the tabs (used for screws) on the SIR-GM1 unit up. Now it will fit in the console.
Once you have it where you want, connect the pins. I couldn't find the part number for the plastic piece that connects to the car so make sure you use terminals that will fit each wire in securely.
After you have confirmed that each is in the correct position and secure, connect the antenna and confirm that the unit does work. Pressing the band button will get you to SIR1. You should get some kind of a message on the head unit that tells you to call and activate your radio. The message will be visual (may need to press the PTY button) as well as audible.
If the radio works, use electrical tape to secure the wires into the harness. If someone knows how to get the correct connector please let me know. But if you are smart with the tape, it will hold up just fine.
The antenna installs just as it says in the installation paperwork that comes with the SIR-GM1 (the Sirius website has a better explanation on how to run the antenna) . Run it from the roof and down the back window (rear left of the car). I found that there was a wire group in the rear left that came out beneath the rear left passenger seat. The seat easily lifts out of the car so that you can route the wire to the center console. Just make sure that when you remove the seat that you are careful to disconnect the connection for the heated seats (its under the right rear passenger seat).
I hope this helps! And again, if anyone knows how to get the female connector piece please let me know. One other thing, I routed my wire under the molding near the door then up under the drivers seat. I think it may be easier to go right up the middle under the carpet but I didn't want to pull it up. You may be able to pull the carpet up and go that way, but I'm not sure if the adhesive in the carpet will re-stick.
