Sears sells a really great radio, gets excellent reception, uses a 19.2 volt drill battery, it will run for 14 hours continually, on a single one hour charge. It sounds good, even in a package car that rattles. It's small, and bungee cords down to the dash really well. They are about 30 bucks, I'd say spend the extra 3 bucks for the 3 year Craftsman warrenty, and use your drill battery, or buy one at Sears. They are a real good deal.
For years I used a radio shack radio with a power source that plugged into the lighter of a sprinter. The Craftsman radio is so much better. No wires, sounds better, smaller.
I wouldn't mess with the wiring on a package car. Most of the package cars' wiring are not much to brag about, without having you try to tap into it. And, I noticed a memo about six months ago to the mechanics to remove any and all power jacks, non-factory lighter power sources, or any unapproved wiring modifications. I believe they have done that in our building to a couple cars where the drivers had installed a power plug.