Charging/shortcut problem in Ford Transit?

Hello,

I have a Ford Transit 2.5 Diesel 1990 and it has developed a strange electrical problem...

First of all: charging light is on when driving (not in full brightness, if it makes a difference).

Second: one wire connected to the ignition switch becomes very hot and it is melting a plastic connector. The wire is black and light gray and a bit thicker than normal wires.

Other than this, the car is working fine. It starts and drives ok at least for now. This problem has only been for a couple of days, so I'm not sure if the batteries are charged or not.

Sounds like a shortcut to me, but were can I start looking for the problem?

Thanks in advance for any help.