The add a circuit is your best bet. You would need to locate the fuse for the "cigar lighter" as that usually turns off with the ignition. As for running the wire in the headliner, just tuck the wire in where it meets the windshield. If it keeps falling out, put some tape or a small zip tie then tuck it back in.
2015 Billet Silver
2012 300SRT, 2009 Jeep SRT, 2006 Charger SRT
Too many mods to list for each, so let's just say a lot was done over the years.