Tail Light Switch problems of the 2004 BMW X5

The tail light assemblies (brake lights and running lights) has failed on this vehicle three times( the left assembly and once on the right). All failures were due to corrosion (vehicle has never been in an accident). The left assembly failed in November 2006 (at approximately 42,000 miles) and again in March 2011 (at approximately 110,000 miles). The right assembly faild in February 2009 (at approximately 68,000 miles). These failueres are not light bulbs that burnt out but complete failure of the housing that holds the light bulbs. The problem is intermitent. When the dashboard messge indicates "check brake lights" or "check rear lights" and I do the lights appear to work fine (all ligts on when switched on or when brakes applied). Message gos off and may stay off for several days and then returns and lights up with every application of the brakes. There are several similar cars in use and all of them have experienced the same problem with the tail light assemblies.