You will need to connect all 3 wires in the plug on the 20v alt.
There should be a sticker on the alt as i can't remember which is what at the moment. They will be L = charge light
I = ignitin feed
S = battery 12v
to chech you have the correct wire on the car, turn on the ignition and using a test light ground the wire, if it brings on the charge light..you found the correct wire.
will have a look at an alt in the morning and tell you the positions on the alt.
cheers