Oh my God, I just got one from OReilleys and it's doing the same fricken thing. I've seen some people complain about the SAME issue but with no real fixes.

1989 F150 Headlight Switch Problem - alt.autos.ford | Google Groups

check this out..

someone once told me that the switch switches grounds, if this is true I might have a theory.. but if it switches 12v, I have no answers.