MD, there is no switch. I wired them into the parking/tail circuit.
Frank, I only fitted 3 LEDS in each slot because at the time (nearly 2 years ago) Ultra-bright LEDs were very expensive. 6 of them cost around £20!! but there are quite a few cheaper (and more expensive options available now that I'd like to try. Have a look at this stuff for some ideas;
http://www.lumileds.com/products/