It can be as easy as simply adding a little child and a transient (normally open) switch. Look for the sign "hot" exhaust - is a cable that goes from the center of the leg volume control, and ends at the end "tab on the outlet. Now, add a cable, for example, during the volume control, and welding of this cable to switch (either legs up). Then another wire bond for the remaining leg (the two who want to use) the switch and solder the other end to ground.