You can very cheaply add one of the Kenwood, JVC, or Audiovox Sirius tuners to your exisiting head unit. I have an old Clarion unit. If they still make a Sirus add on piece, buy it, as Clarion is by far the best of the aforementioned 12volt audio companies.
Pick out the tuner that looks best to you (they differ greatly) and then buy one of the three varying kits available:
Tape Cassete Kit: Uses a wired cassete that pumps the audio through the oem system and sound pretty good, but leaves wires laying around.
RF Modulator Kit: Doesn't sound quite as good as the cassette adaptor, but is a much cleaner install.
Wireless RF blaster: Is the cleanest and easiest install, but exhibits the poorest sound quality of the three.
Adding Sirius to your existing system can be done a few different ways, and all don't involve the loss of your steering wheel controls.
BTW, sirius is very cool, and worth the investment. I enjoy mine everday.