Normally when you have one dim headlight it is due to a bad connection, either power or ground. In my experience it is almost always poor ground. However, that won't make your headlights fail. I think you have purchased some poor quality bulbs from your vendor. If your next one fails and it is the same brand, don' t give up, try another bulb from another vendor, but stay away from the Chinese bulbs. My current M35a3 has the same headlight bulbs in it that it had when it came from the Army, over eleven years ago.