is there condensation in the lens? corrosion in the housing? Just a thought. I use to have a 00 DII (what a great truck!) that had good 'oll H2O building up from a bad seal on the back side of the housing.
Kept blowing bulbs untill I bought a new lens. I dono, worth checking out
"Out of every one hundred men, ten should not even be there, eighty are just targets, nine are the real fighters, and we are lucky to have them for they make the battle. Ah, but the one, one is a warrior, and he will bring the others back"