this means the lamp and blue wire are OK so the problem lies at the alternator:

1, Brushes are worn out
2, Regulator/diodes broken
3, Alternator body not earthed

if it's 1, or 2, then you're best just changing the alternator (no offence, but from the questions you are asking it's unlikely you will be repairing the internals..?)
3, is fairly unlikely but may be worth checking continuity from the engine block to alternator body.