All the other terminals are output, even the "W" where fitted(single phase tapping)
What you need to do is make sure the alt is charging, oddly enough it still can even with the light on, depending where the fault if any lies, it can also not charge with the light off as well, revving above 4000rpm should be enough to initiate charging even without blue wire connected, if it doesn't it is duff, just put a metre on there, you will see an increase when the engine is running, if charging.
tabetha