the breaklights and break switch on my 2002 mk4 golf tdi keep blowing.

first failing in dec 06, they have subsequently blown in may, june and sep 07, with the engine warning symbol lighting on the dashboard. the vw dealer has said each time that they will replace bulbs and switch and that should be the problem resolved. what they haven't worked out is why it fails again 2 months later....

any help getting to the root cause would be much appreciated.