Originally Posted by DazC

A Ford MUST have a battery warning lamp other wise it will not charge.

The basic circuit is a live ignition feed to the bulb. Then from the bulb it goes down that blue wire to the back of the alternator in which it gets it's earth. Once the alternator has been excited, the voltage will increase on the terminal which the blue wire is connected to, and basically block the return path to earth for the power flowing through the bulb.

You either have a fooked alternator, broken blue wire, bad connection or a blown bulb.