Whats the light on the back of my Hyundai?

My boyfriend has a hyundai accent 2000 and i have a hyundai excel, both have a matching little light on the right handside of the back of the car. Just wondering what the function is because i have found no buttons in my car that turn it on, but in my boyfriends car the rear demister button turns the light on (we believe this may be due to someone rewiring it though).