[ree]

Since your headlamps are always on, I don't see how it could be fuse related.

If the sensor on the dash is indeed clean and you weren't just spreading gunk around on it, then it sounds like it's gone bad. I assume it's using a photoresistor that reduces resistance when light hits it. In this case a bad one will probably make the truck think it's dark out. T'is speculation, however. I suppose it could be faulty wiring to the sensor as well.