Most of the auto headlamp Add-ons I have seen just use a relay that gets constant powre from the battery and a switched lead when the car is on. You typically just jump the withe from the relay onto the headlamp power and it's ready to go. (Assuming you got the diode installed in a good location.) You never have to mess with the switch and all the witing is under the "hood". (Excepting the lead to the battery in the case of the Smart.
It's typically a 15 minute install.