Next step, I tapered the knob on the center potentiometer, which is the one in the stock location. This allows the stock slider knob to snap right in place. I also removed the white center pieces:

This is the factory knob, notice the cutout on this side? I will use the cutout to link together my two potentiometers.

Here's the knob in place on the center potentiometer, and a dowel connecting both potentiometers together. I afterwards used hot glue to help secure the pieces together and keep the dowel in place:

Next part is to wire up the overhead unit. I used black wires for Terminal 2, which is the slider itself. Red wires short out to Terminal 2 when the volume is at zero and the green wires short to Terminal 2 when the volume is at full.

Here's the bar re-installed with my new volume control wiring in place: