WARNING: Modifying certain things may be against the law in your area. I really don't think this has any legal issues (even in up-tight Massachusetts), so you should be good.
Also, please note that access to wiring in your vehicle may not be as easy as mine. I drive a Dodge Neon, so all you have to do is reach under the dash, and wallah! Wires a-plentiful. Nicer vehicles, like Mercedes-Benz's or BMW's might actually do a good job preventing you from touching the interior wiring. You should have a pretty good understanding of vehicle wiring before attempting this Instructable, but I will cover the basics.
IMPORTAINT: You will be modifing your vehicles wiring. MAKE DAMN SURE you know what you are getting in to. Your vehicle may have extra wires like mine did, so this won't be a big issue, but it may not, in which case you will have to do a little more work.