e36s have an array of different problems with brake lights, but a brake light circuit warning on the computer usually means that the brake light switch on the brake pedal arm is failing.
Park where you can see your brake lights shining on a wall in your rear view mirror. Then press the brake 10 times. If the brakes do not illuminate each of the ten times, it is the brake light switch on the pedal.
The easiest way to get the old one out is to crush it to bits with a large pair of pliers. You may have to straighten the bracket just a bit after it is out. Then the new one snaps right in.
Don't buy a cheap Chinese 'white box' switch or you will be replacing it again in a year. A good one is about $15 on line or $30 at the dealer.