Recently I had to upgrade my six year old XM passport in my 82 LeBaron ragtop to this years rendition which like the first unit plugs into cigarette lighter for 12v power. Normally when not using this car, I turn the battery disconnect to deactivate its electrical system, as this car will kill a battery in three weeks of nonuse; something it has done since taking delivery in 1982.
The other day, after the car sat parked for about a week, it almost didnt turn over when starting it for a short trip. I though this is odd, new battery less than a year old, only parked for a week, currently in everyday use so no battery disconnect needed
After charging battery for six hours back up to full charge, I had to find the current drain causing the problem. It was the XM passport replacement. When it is turned off, the screen indicates it is powered down then goes black, however a test light between battery and ground indicated current flow with a bright light. Turns out when Passport is off, it aint fully off, and continues to draw power. Now I have to pull its plug from the lighter.
I suspect that Sirius uses the same technology as XMs portable receivers.
As for power to the lighter in your car, someone could have disconnected it to power a sound system. Trace its wire back to the fuse block, as that device should be seeing power when the car is not running, and ignition key in off position. Check for tightness of your lighter socket to dash, as this is its ground connection.
Also generic lighter assemblies are available in any part store electrical aisle if your unit is not working.