I've got a 2000 S also.
Antenna in my Sirius is on the dash. Right most corner. It plugs in on the other end to the Sirius receiver. I have the plug and play kind that I hook up to my PCs when I'm not in the car. Its a very easy install.
For your XM head unit it will most likey connect to a rear jack and your done. You'll need the pins to pull the headunit out, othewise its off to the sound shop.
p.s. here in NJ, Circuit City was going to charge me $80 to install the antenna.
Total rip off.
Sirius PNP3