double check this as i did it a looooong time ago, I put footwell lighting in my 350, take off the A pillar cover on the passenger side and you will find a pink wire this can be used as a trigger, DO NOT overload this, use it as a trigger to switch a solenoid, this wire will work as the door opens
Your other choice is to tap into the door switch itself, these are usually a switched earth, tap into a live and this to switch the lights on/off as the door is opened