Volvo: 1997 850 T5 Intermittent Center Brake Light Problem, brake light switch, bulb failure

Question
The center brake light (1997 850 T5) has an intermittent problem.  It will work.  Then it will not.  When it is not the warning light comes on.  The problem can occur whether or not the car is moving or bouncing about.  So I have ruled out wire moving problem.  I have ruled out bulb failure.  Could a faulty brake light failure sensor or brake light switch create this problem? Somewhere I have read that the center brake light does not go through the same brake light failure sensor that the right and left brake lights go through.  Is this true?  

Have you encountered this and what might the problem and solution be?

Thank you.

Answer
Scott,
Usually they work or they don't. So that said testing is in order. I would follow the wire to the light in question and hook in a test light that can be montiored while driving and stopping. Then test drive and watch to see if the test light also fails when the brake light does. If the brake light fails and the test light works then look for grounds for that light. If there is no power follow the wire closer toward the bulb failure relay. The third brake light does in fact use the same bulb failure relay as the rest of the lights.  Located in the trunk area. A relay about 4 by 5 inches They all do use their own seprate wire in and wire out points.