The cigarette lighter is 24V.
Depending on the XM unit, most have a plug that will handle 12V or 24V.
If you need other,
http://www.voltageconverters.com/inverters_110v.html
this place has 24V to 110V converters, then you would need a 110V to 12V adapter, which is pretty readily available.
BTW, which one do you have??