I use the XM radio in my Baron all the time, and it works great for those
long cross countries. I had all of the cigarette lighters converted to 12
volts, so the existing plug worked fine. Radio Shack sells a 6 volt
converter from 24 volts as well, and it's pretty cheap.

One thing I found initially was a lot of hiss and noise, and this was
completely solved by inserting a ground loop interrupt in the audio circuit.
The resulting sound is now excellent!