That should be fairly straight forward
Take that wire you previously added off the output jack. Look at the volume control, there are two wired tags there, one goes to the jack, and the other goes to the switch. Solder the wire to the side that goes to the switch. At the moment the signal goes straight to the jack bypassing the volume. This will now allow the signal to flow through the volume to the jack, thus allowing you to adjust volume and the pickup will always be on, i.e. irrespective of where you put the switch that pickup will always work and the others will be added according to switch position.
alternatively just put a wire from 1 to A, removing your other wire first of course
http://www.premierguitar.com/ext/resources/archives/17b0a9ee-6af3-4c39-ac35-48bf361cb3a1.JPG