the current to the little little bulb in the cigarette lighter wouldn't be enough to run a set of neons....maybe an extra couple of the small bulbs...or maybe leds, but probably not much more...
your best bet would be to hook up your neons to the current supplied to the actual cigarette lighter....then hook up a switch to it too. This way, you have the benefits of having a fused current, and you also don't have to worry about leaving the neons on because they will turn off when you take your key out.