First off it's a common problem. It can be traced to one of two things, both of which are, again, common on E36's: Wiring on left trunk hinge. It's wrapped in a plastic conduit but the constant use of the trunk wears the wires insulation off to a point where wires begin to touch. Other problem is with the break light switch located under the brake pedal. This is an easy swap out and can often be attributed to the 'break light failure' warning.