Well, if you don't want to drill a hole in your firewall, you can run the power wire (if is greater than around 4 gauge) through your fender. Open your door and pop your hood and you should see some light coming through the door jam from a hole where the hood latches to the car. You can take a clothes hanger and run wire through that. If you want a cleaner look your going to need to drill a hole or use an unused grommet. If you are drilling a hole, take a hammer and nail or something with a point and dent in where you want your hole to be in your cabin, then go to your engine and look to make sure that you aren't drilling anything important... if you are, then just do it again in a different spot until you get somewhere that isn't going to screw anything up.