Rear View Camera Windstar

Trying to get this working. I tied the positive of the camera to the positive of the license plate light and the negative to ground. The idea here was that the light would come on when I turned the headlights on (plate lights light up). But what I found was that the camera was always energized, which would wind down the battery over time. Apparently, the way Ford makes this and the rear reverse lights also, is that the negative goes to ground, thereby constantly keeping the positive energized. Someone else experienced problems getting the reverse lights as the trigger for the camera, they also found the camera was always on. So I'm trying to figure out how to power this camera so that it is off when the car is off. Any ideas?