Help

I drive a BMW 520i and recently i started having problems with my electrics.

My airbag light comes on which means my stereo wont work, the light goes out whilst im driving and the stereo comes on, or my interior lights comes on whilst i am driving along.

The other strange thing is, if my airbag light comes on, when i press my brakes it dims.

What does my airbag panel light, the stereo, the interior lights and my brakes have in comon?