Depending on the Sirius unit you get, you could get one with built in FM modulation and all you would need to do is stick a antenna on your roof, plug it in to your cigarette light and stick the thing to your dash. Its not the cleanest sounding thing out there though, but it works.
I only use the fm modulator when I drive a work truck (no tape/cdplayers only radio) and its sometimes a pain. If you only use it locally and keep it tuned to a empty station your fine, but on long trips that i take, i have to swithc stations every once in a while.
Or, if you want it to sound really nice but use your stock radio, get one of these.
http://www.crutchfield.com/S-2sIXOMm...0&I=142FMMOD01
or
if you get a audiovox sirius reciever, you could maybe use this.
http://www.crutchfield.com/S-2sIXOMm...50&I=220SATSWB