knob above radio..

[MvCe30]

Hey Guys,
Recently got to purchase my dream E30.
Q's,
Above the aftermarket radio there is a knob, turned it and it doesn't do anything
(possibly broken inside the dash) tried to google it and no luck..

Anyone have something like this is theirs and know what it does?

Thanks!

[Codym42]

It's for a subwoofer. Do you have any amps in your car?

[MvCe30]

Thank you! Yes I do, there is one in the trunk.

The previous owner did a bunch of work and mod's.