Help to remove a Business radio on an 1999 E39

When I remove the volume button, the only thing I see is a clear round
plastic cover of a circuit of some-kind, and right above the button spike,
there is a small hole (about 3 or 4 mm wide) with a vertical groove (sorry
if my English isn't so good as I wish!) I've already inserted a small
screwdriver in it and tried to turn it 1/4 turn, but it doesn't turn...
Any thoughts? Shoudn't there be a 2mm hex hole somewhere? Please help...

All the best,

Vasco Ricardo,
99 E39 520i