It's not the earth wire, as it is working the starter ok, as said the B+ is the battery output from the alternator, check around the battery carefully as ford had the smart idea of running LIVE wires within a black sheathing and sometimes the wire is a bit short inside the sheathing and people don't notice it is a red wire inside the black sheathing and so put it on the wrong terminal.
You need to measure the actual voltage at rest and when running, could even then temporarily run a wire from the B+ to the battery pos and see if it then reads a charge if it doesn't the alt is knackered, even though the light goes out.
tabetha