wired my girlfriends to a SPDT relay (you can pick these up at a local auto parts store for like $2-5) so that they come on only when the parking lights are on (which are on also when you turn on the headlights).
if you wanted to do something like this, you'd wire the white wire noted in the picture to pin 85, a constant +12v source to pin 86 and 87, and the power for the leds to pin 30, and then ground the leds independently. you can also add a toggle switch inline with the power source to interrupt this relay.