Trunk lock broke - how to open the trunk?

My trunk lock didn't open properly for months, but now it's completely gone. I can turn the key but there's no indication about the lock mechanism stuff turning aswell; there's no resistance. I've read it's possible to open the trunk from the inside. My problem is, my car, 1974 Plymouth Valiant 4 door sedan, has a bench in the back. I have no idea how to remove the bench. Is there another option to go in, like from the license plate light?