Headlight Problems

[1972GMCsierra4x4]

So my headlights have been slowly going out one by one. I started out not having any lights on my dim circuit with all four lights coming on when I switched to brights. Then a few days later, only the punter two lights worked and as of yesterday I have only one headlight, my drivers side outer. My mechanic said I need four new bulbs, but installed all four of the ones I have now about a year or year and a half ago. What could be causing this out of the blue. I have replaced my dimmer switch and that didn't help. Any ideas?





72 GMC Sierra K-2500, 350 with 600 cfm edelbrock and edelbrock performer intake. D44 front and D60 rear