If you have a sat radio tuner that is made for a specific aftermarket head unit, you may be able to get an infrared transmitter for your head unit that can be connected to the steering wheel controls. The transmitter then works like a remote control for the head unit. I had one of these in my Mitsubishi Eclipse with an Alpine head unit and the Sirius Alpine tuner. Of course, in order for this to work, your head unit needs to have an infrared sensor; if your head unit didn't come with a remote, then the unit probably doesn't have the sensor.
If you have a plug-n-play sat radio tuner that comes with a remote, you may be able to find an infrared transmitter that will work with that tuner. I'm also pretty sure that you can get tuners that you can program with your current remote -- kind of like "teaching" a universal remote for your TV or DVD player. I'm not 100% certain of that, but I do seem to remember hearing about them when I was replacing the head unit in the Eclipse.