if youve got an aftermarket radio all you should need to do is switch the red power wire into the head unit so its running off a constant source of power (best would be tapped directly from the battery w/ an inline fuse, theres an open grommet thru the firewall behind the dash to make this easier) or easier, but probably a little more risky would be tapping the yellow wire into the head unit

will be the same with a stock radio too, except i dont know what color the wire you need to get constant power to will be