First off, I would like a more experienced auto electric person to give my plan a sanity check.

Assuming you can still open the door and pop the hood open….

Important - First go to the positive terminal on the back side of the fuse box. With an ohm meter test for continuity between the positive terminal and car chassis. If you read a low resistance then it’s possible that the positive battery clamp is contacting the trunk floor, or other structure, and you should abort this method of powering the electrical system.

If, on the other hand, your resistance test indicates an open circuit then it should be OK to connect +12v to the positive fuse box terminal and -12v the the car chassis. That should provide the necessary electrical power to unlock the trunk.

Note: It’s quite possible that you will never read a totally open circuit when doing the resistance test due to things like the clock etc that constantly draw juice. But I’m not going to stick my neck out and suggest what a “safe” level of current draw would be in order to safely connect power to the fuse box terminal. Hopefully someone else can shine some light on this.