Do you have a police pursuit model of the Charger? They have a built-in taillight flasher. Currently on the 2011's they alternate the brake with the reverse lights, but they are going to be issuing a software flash for that because flashing white to the rear violates SAE. They're going to be changing it to just flash the 2 main brake lights. No date for that flash yet, probably a few months.
If you don't have the Pursuit model (the "Enforcer"), then you're outta luck, unless someone knows a way to do it that I'm not aware of. It'd be easier to put a couple Vertex's or something in the rear light housings.