Boost control knob

Hello all, new to the group and a new owner of a used 2011 Tundra 5.7l that's had the Magnuson TRD supercharger installed. This was already installed when I bought the rig and I'm becoming familiar with it. Fun truck to drive however I have a question about the boost control knob. It's a knob that the outer end and lower portion are adjustable separately and I'm wondering which portion does what? Any input is most appreciated!

 

That’s for your stereo.

 

Seriously?! The previous owner had an amp and sub in at one time but left the wiring which I removed. There were accessories that had been removed but the wiring was still hot which pissed me off. That's pretty funny, this whole time I was under the assumption it was for the supercharger.

 

So I just got to rooting around under the dash and that knob is wired to a small box adhered to the back of the dash. Harness leaves the box and runs to a connector plug in the factory harness. Pull the little box and discover that its a brake controller for trailer brakes. Never have seen a set up quite like that before.