I've left it open for long enough to have it time out and shut off.I believe the rear cargo light comes on when the tailgate is open, so most likely it would drain the battery. I have never left it open long enough to see if it would time out and shut off. I don't believe there is a shutoff switch. You could try leaving the tailgate open for 15-20 minutes and see if the light goes out. If it doesn't, then you have the answer you need.