I tried that connecting the leds to the a/c wire from the stock lights and they will flicker and drive you nuts. I just wired the 6 led 18w to the battery with a inline fuse and a rocker switch and they are bright. I am only running one for now.
This our first trip with one led 18w straight to the battery with a switch.
http://www.youtube.com/watch?v=EmZz6EbgB1c