You buy cheap bulbs?   ;-)

Seriously, get a cheap digital voltmeter (preferably one that stores
peak voltage) and monitor the actual voltage in the car's system. You
may need a voltage regulator. Or, check to be sure that the bulb is
being securely held in place. If the bulb is allowed to vibrate in the
socket it will dramatically shorten its life.

            __  __
    Randy & \ \/ /alerie's
             \__/olvos
'90 245 Estate  -  '93 965 Estate
  "Shelby"     &      "Kate"