I'm not a mechanic and I don't own a 200 3 Ford Excursion
I would assume that you could take a + or - trigger off of the Neutral Safety Switch on the shifter that prevents the vehicle from being started if not in park if automatic or neutral if standard, see http://www.car-stuff.com/2003fordexc...fetyswitch.htm for a picture of the switch. you would need a wiring diagram for the pinout on the electrical connector.
AMP Research has a kit you can buy from http://www.runningboardworld.com/amp...2002-2003.aspx, but it hooks up to the existing wiring harness , and is activated when you open the door. Sometimes, such as when your parked on the side of the street,and you have your back to the truck opening the door, this would cause the step to possibly push you out into traffic when it extends.
My thoughts would be to use the neautral safty switch to only allow to be activated when the transmission is in park or neutral, but install a remote control so you can extend the step when your approaching the truck and retract when the transmisson is moved from the park or neutral position.