I've had Sirius for almost a year, and I hardly buy CDs anymore. In my PT I still have the stock (although decent quality) head unit. I use the direct FM input with mine. Basically it connects in between your antenna plug and the radio. Negates any effect of outside frequencies interfering with your sound. I tried using the included antenna (sucked), a tape cassette adapter (not bad), and finally went to the direct FM input module. Its a little black box the size of a relay, and you just plug and play.
I have one of the plug & play Sirius units, and I have the boombox inside that I use. Also, with Sirius, you can stream CD quality sound from their website if you are a subscriber.