I had the exact same problem. There are actually two plastic pieces in the back - one with a rubber gasket around it that is supposed to "seal" the light around the window and the one that clips up - the one that clips is supposed to hold the one with the gasket in place. At least I think that's how it works.
Anyway, mine was clipped up all the way but the gasket wasn't touching the window all the way around the light, so the light was leaking into the back seat. I went to Home Depot and spent about $5 on a roll of foam tape (I think it was 1/2" thick). I unclipped the 2nd piece and installed the foam tape so that when you clipped it back in, it pushed up much harder on the piece with the gasket. Problem solved.