The wire umbilical that connects the hatch to the body is a well known problem area. With wires inside that crack, short and open up
Use electronic solder [that has lead in it] not plumbing solder and use shrink tubing for all your repairs
This is fixing it properly. I think that from the sound of things you have another harness that you can scavenge additional wire from.
For instance for the umbilical wires making repairs where the breaks occur mighty not be so good.
Use scavenged wire to bridge across the umbilical. Yes that will require to repair two joints, but I think is the way to go.
Any sign of melted wire is not the cause but rather a symptom of a deeper problem