Illumination Lights Not Working 85 Foxbody

You likely need a new headlight switch, disconnect the battery, pull the nob out and feel around under the dash for the switch and locate a small button, this releases the nob, unscrew the ring that holds the switch to the dash and it will come out far enought to unplug
You may be able to take some electrical spray and clean the spring and connections but a new switch is not expensive.