Brake light circuit warning usually means the brake light switch is either
bad or stuck.  I had that problem on my 97 M3.  A spray of silicon on the
switch (located under the driver side lower dash panel above the brake
pedal) cleared the problem up.  If the spray doesn't work, the switch will
probably need to be replaced.

Kyle.

The switch is very cheap, less than $10 and takes less than a half  hour to
replace.

Find a new mechanic, the one you have now didn't even use lube when he bent
you over. This is a common E36 problem, and should have been recognized as
such. You should buy an Owners Manual from the dealer, and a "Bentley Repair
Manual" from Amazon.com. The Bentley will provide a lot of really useful
information even if you don't do any work on your car.

It's Brake Light Circuit Error. This error is caused by a faulty switch that
is mounted tothe brake pedal. This error is intermittant for most of us, but
when it comes on, the only way to clear it is to cycle the ignition off then
back on. Once it comes on after the car has been started, it will remain on.
The next time the car is started, the error may or may not return, depending
on the amount of dirt and crud that has accumulated on the switch contacts.