Adaptive Equipment problems of the 2007 Ford Focus

Rear taillight bulb blows out as if an electrical short in wiring. Bulb will look black, even the socket has black discoloration. This has been an on-going problem since we purchased car in 2009 with both the left and right rear tail lights. To date I have replaced about 15-20 bulbs to the extent I purchased two (2) boxes of bulbs a socket wrench and correct socket to keep in car to replace bulbs each time it happens.