I had a similar problem on my 1990 Tbird, and it turned out to be the dimmer switch, which is also the headlight switch. I pulled it off and then opened the switch and what it was a circular spring that was a resistor, and then as you rotated the dimmer switch, it moved a metal contact over the spring to change the resistance. On mine the metal contact had worn down and lost contact with it. After cleaning and a little bending, it was good as new.
If your fuses check out, and simply turning the dimmer switch doesn't fix it, its pretty easy to pull it out and look at, so I'd do that next.