I got one of these: Amazon.com: Megabrite: Wireless - Motion Sensor - Undercabinet LED Lights: Home Improvement Covered it in black tape (though not really necessary). Velcro'd it on the vertical surface of the interior of the lift gate. Advantages:  No wiring needed.  Motion sensor turns on when the trunk is opened.  Really, really bright.  Where I've mounted it, it can't be seen from inside the car or really even when the lift gate is open; the black tape just makes it even less evident.