Side box cable

Raise the mattress,drill a hole above the latch then use a jigsaw to cut a hole big enough to get your hand in to open the latch.I have had to do this on 2 of them.

 

IIRC, the cable also has a clamp behind the side wall covering that holds the sheath, preventing you from pulling it through and fishing the replacement down. So you'll need to remove the interior trim and side wall to get the replacement done. If you remove the side wall, you might be able to get it open by pulling on the cable sheath on the far side of that clamp without having to drill / cut a hole to get your hand in to the latch.