Since a few of you PM'd me about XM radio related questions, allow me to break up the mystery (fairly simple).

For your car (any Scion for that matter) you need the following
SkyFi (or Roady units)
Car kit (car cradle, wires, adapter, etc)
Wireless (cigarette lighter plug) or wired (behind the dash, in line with your antenna) FM modulator.
The wireless is the quickest install, no hardwiring at all, but the wired has better sound (again, directly plugged into the back of your factory deck). Wireless on the other hand, can be removed within a few seconds, which is great for people who need to take the complete system out quickly (selling your car, trade in, servicing, whatever).
The price-dealers charge around 500 bucks installed, where the parts only cost between 120-150 (depending on which kit you go with, wireless or wired). Install can be done by yourself (if you have 2% mechanical savvy, and can follow instructions), or you can have your local stereo shop install it for 40-60 bucks. Please feel free to e-mail me or PM me if you have any questions, I am here to be of help
Alex