Interior lights not on w/dvd player

I bought my '06 Equinox and it had a DVD player mounted where the dome light would be about the back seat. It has lights in it and work with its switch, but the lights will not automatically turn on when you unlock the car or open the door to help you see. I have a feeling the P.O. didn't hook up something. There are four wires in the harness (12v and gnd. and then 2 white ones), so what do they do?