The light dimmer controls the light intensity of the dash cluster and climate controls, not the dome light. You could relocate the dimmer to the other side (where cruise control is/would go) and put in another knob for the subs. Or, just put in a knob on the cruise control side for the subs. You could also put some kind of slider near the hand brake (in one of the removable rectangle tabs in the console). So many options, just be creative.
As for a DIY, you can find info on the knobs by looking for a push to start button (I believe they show how to remove it within). Wiring up a knob should be rather easy, though.