Hi Marcus and
to the forum.
It is highly unlikely to be a melted power cable, unless you have been pushing a lot more than 100 Amps into your pie.
I suspect it will simply be either a broken wire or a poorly soldered joint, but it might also be worth checking the battery output first to make sure it isn't a poor battery power connection or the switch contacts themselves that have failed in addition to your original fault.
If the fault is in the motor harness, you will have to carefully cut open the molded rubber body of the junction unit with a very sharp knife in order to gain access to the wiring inside. Make sure your battery is disconnected first!
I have never cut one of these junction assemblies open, but I would expect to find a small circuit board with all the wires soldered to it.
Please let us know what you find.
Alan