There is a long discussion and possible fix at
http://forums.bimmerforums.com/forum/index.php  Seems that the  wiring
harness gets worn in some cases and causes that problem among a few
others.  The harness goes from the trunk and into the body of the car
near the left hinge and through repeated flexing it get worn and can
even cause a fire.  The harness has an additional covering there for
protection and has to be moved out of the way to see If there is damage
[many pictures of damage at that site]  Most people had some kind of
damage after looking.  BMW know of the problem and sells new wiring
harness, fixes can be seen at that site.  You'll have to look around to
find it, use the search engine.

William,

On my car, I had a similar problem with the brake light. After days and
weeks of troubleshooting, I tracked the problem to corroded contact
area. If you look at the areas where contacts are made in taillight
assembly, you might come to discover that there are holes around the
contact areas. I am not sure if you will have the same problem, but from
what you describe, it seems like an intermittent connection issue.

Best regards