ok, here we go..
go to radio shack and buy this:
5 volt regulator IC #7805($.25 at radio shack):
then you have to cut the car charger wire you have, cut close to the cig light base.
strip back the insulation, solder the red wire to the right leg of the IC (the right leg is when the IC is on the table label up as in picture)
solder the black (ground) to the center of the IC
run wire from what ever source in the car (switched cig lighter, or any 12 volt power) and solder to the left leg, solder second ground from the center leg to a ground point on the car (ground wire on cig lighter, or metal frame for example) make sure that NONE of the 3 wires touch, I use shrink wrap to cover the solder points.
now the IC will generate heat (it's pulling off the extra 7ish volts. so I suggest bolting it to the metal frame and keeping it away from plastic or carpet. you can also wrap it in heat transfer insulation (any hardware store in the plumbing/heating dept). just keep it away from melt-able or flammable objects. wrapping in heat insulation and stuffed in an old Altoid can is what I did with mine (taped the lid shut) and stuffed it behind the console.