the slot is useless tbh, you see the trim that goes up the side of the boot and along the bottom (black plastic) just pull that away from the side as its on clips. then it will free the carpet behind it. This will allow you to pull the carper out of the way, which in turn allows you to see the back of the tail light. theres two nuts, undo these and the rear connection and the whole unit just pulls off. This will allow acces to the rear bulb.
make sense?