E30 Zone

Huge problem with brake lights, when brake pedal pressed brake lights don't work, obc and radio go off and come back on when pedal release. This only happens sometimes not always.most times brake lights work fine and obc and radio ok.have changed the brake light switch but no luck

Sounds like you have an intermittant short somewhere in the wires going from the brake switch to the lights at the rear which is causing a sudden current drain.
This will not be easy to find....you will need to physically trace the wires to look for any damage.

First check that there are no problems with the leads in the area of the pedals...pedals moving up and down could be scrapping on the wires.

Yea agree with previous also if there's a bad earth it can draw an earth from other electrical units I had that with my old car but I'd check the first option first as seems more likely

IIRC there's a known corrosion problem with the rear lights earth point, behind the rear bumber IIRC again.