Okay, look at it this way: when the switch is to the left, the new lights get fed the same power going to the stock lights. When you flip it to the right, the power flows only to the LEDs, but not the stock lights.
You could run the one wire from the stock lights, but if you want the switch in the dash, you'll be running a wire all the way to the front, just to run another back to the light. Be easier to tap it off under the dash, so you only have to run a single wire back to the new lights.
Also, you should put a fuse in the constant lead, for safety.
Here's an updated diagram, should make more sense:
Last edited by Soundy; 02-03-2013 at 08:46 PM..