Quick recap on the theory: When the ignition is on, but engine off, 12v is supplied to the dash warning light from the loom (green wire). 0v is supplied from the alternator (brown/yellow wire) so there is a potential difference of 12v-0v = 12v and the LED lights. With the engine running 12v is supplied from the alternator. As a result the potential difference is 12v-12v = 0v and the LED goes out. With that in mind: To check the warning light wiring. Remove the brown/yellow warning light wire from the alternator. With the ignition on, first 'earth' the wire. The warning LED should light. With the ignition still on, apply 12v to the wire (touch onto the main alternator post). The warning LED should go out. If it doesn't the issue is in the warning lamp wiring. If all is okay, leave the brown/yellow wire disconnected from the alternator and do the following. With the ignition on, but engine not running, you should get a voltmeter reading of 0v on the alternator's 'L' pin. Next start the engine, rev it, then check again. It should now be 12v. If this isn't the case it would suggest there's an issue with the lamp feed on the alternator.