If the brights work and the dims do not, then the probable cause is a burned out headlight. A flip up light usually takes a H6054 sealed beam headlight. There are 2 filaments in them. One is for the low beams and the other for high beams. Usually the low beam burns out because it's the one that is used the most often. Occasionally both bulbs will lose the low beam at the same time. More often then not, the driver just hasn't noticed one of the bulbs has burned out. The other cause of the low beams not coming on would be a bad dimmer switch. Try replacing the bulbs first. If that doesn't fix the problem, then the culprit is probably a dimmer switch.
