Perhaps the bulb is faulty?Remove the bulb and wire it across a battery to see if it lights to the correct brightness.You could swap it with oil warning light (if fitted)or main beam w/light.hth

One end of the bulb is connected to the switched 12v supply, the other end to the small terminal on the alternator. What should happen is that the bulb lights until the alternator is going fast enough to charge, there is then 12v on the small terminal so you now have 12v on both ends of the bulb so it goes out. I would suspect that it is wired incorrectly or there is a short to something else. Unless the bulb is connected correctly to the alternator it will not charge, as the lamp provides the exiter voltage. Check the voltage on each end of the lamp and then on the alternator small terminal.

It is possible that there is a fault in the wiring from the w/light circuit.Check at bulb with ignition on ,there should be battery volts on white wire.The earth path to the alternator small terminal via the brown/yellow wire needs to be checked,or alternatively run a new wire from the bulbholder (brown /yellow)direct to the small terminal at alternator.Discard existing wiring.This is a basic circuit and there is no need for any diode or additional components.hth