Fixing a Christmas jumper with lights in it

What is it?

A Christmas jumper with a circuit of lights in it.

It has a battery box that takes two CR2032 batteries, giving juice of 6v, connected by a JST connector(?) to the circuit that runs through the jumper.

Problem

The lights in the jumper don't come on.

Diagnosis

Checked that the connector was seated properly. It was.

Tried with a new set of CR2032 batteries. No difference.

Tested the batteries with a multimeter. Showing 3.34 volts, so all good.

Tried checking the continuity of the output from the battery box. Nothing coming out, but maybe just can't make a connection with the multimeter properly.

Done a bit more probing with the continuity tester. Great tool. Basically one of the wires from the battery box isn't working. The crimp looks fine, so it must be a loose connection in the wire itself? Which seems a bit strange, but honestly that what it looks like.

Could replace that wire, but I don't have the tool for redoing the crimp. Probably easier to just buy a battery box.

I decided to trim out the middle bit, solder in a replacement wire, and put some heat shrink on.

So… the battery pack now works. Turn it on and… only the first light comes on! Bah humbug.

I cut a hole in the inner lining of the jumper, and it turns out that the wires seems to have come loose connecting to one of the lights.