Once again, a nice video Nick.
I just love the sound of your Moto Guzzi - could listen to it all day. As you actually do yourself.
About the alternator light - if everything else is working ok, then from the wiring diagram it would appear that the red wire from the light has become earthed somewhere and is therefore making a circuit. It (the red wire) must have become detached from the alternator itself otherwise you'd have much more serious problems.