There are 2 headlight fuses. One for the left light, one for the right one. If one of the two fuses are blown, then one headlight will be dim (yeah, odd, I know....you would think the headlight would be completely dark). Anyway, the fuses should be red 10 amp fuses and will be adjacent to each other. You can check the fuses with a test light, to see if they are blown (headlight fuses have power going through them all the time...as long as the battery is hooked up).
If the fuses check out fine, then it could be that the wiring terminal that the fuse slides into is spread a little too much and isn't "pinching" the fuse tight enough to make a good connection. You can unhook the battery and use a small flat screwdriver and push one side of the wiring terminal towards the other one to get it to "pinch" tighter.
If neither of those work, then you probably have an issue with the wiring somewhere.
I don't have a diagram/pic of the fuse block layout in front of me. But I can probably get you one in the next day or two, if you still want one.