i had that issue on an 88 turbocoupe years ago. i replaced everything imaginable. then i called the dealer and they told me it was a recall issue and the wiring harness in the steering column had to be replaced and was a fire hazzard unless replaced by the dealer under the recall.
i dont know if its still able to be taken to a dealer under recall or not. or if thats even your issue.
i started with the headlight switch. that wasnt it. i then went to the hi/low switch on the column. that wasnt it. it drove me nuts. the headlights would work only on high beam and only if i HELD the switch all the way into the high beam mode.
the recall was a general recall for late 80's fords