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
thats the pass mode when you pull the high beam switch back and hold it. Only the high beams turn on without running lights. Start at the obvious place 1st. Verify that the low beams are in good shape and the light. Check the connecto wiring at the head light then check the headlight switch using a test light or volt meter.