Follow the wire from the trunk lamp back, it will take you to the switch.
It should be pressure trip, or magnetic type switch mounted in the rain gutter for the trunk or at the hinges at or near the body when the trunk is open, (pressure off when hinge moves off "closed position)
If it's magnetic, you may need only adjust the magnet, If it's pressure, you may need adjust the height if it goes off when you push it with your finger. If it won't go off at all, the switch is bad and needs a replacement.
Meantime, I'd pull the fuse or disable the switch, as you will be susceptible to dead battery's after sitting over night if you don't.