I recently found out that Leds are plug and play, all you do is replace your bulbs with led bulbs and go from there. A good place to buy em from or ask questions for your application is http://www.autolumination.com/
good luck hope this helps. ;)
Sorry i noticed after I posted that you meant underdash lights on a switch, you can buy sockets and wire them in if thats what your going for, they could be mounted anywheres. I personally don't like the underdash or under car lights, makes em look cheap, my .02 cents.