Sorry about not getting back. I haven't heard from your either after your last reply. But I will still help you. Again I am sorry.
Since the dome lights work off the door switches and not the switch on the headlight switch, it is not that difficult.
It is either a bad dimmer switch, or wiring. More that likely the dimmer switch.
We can test it by removing the headlight switch/dimmer switch and plugging it back in.
Then with either a test light or a meter, turn the dome lights on with the switch and the doors closed. Check for power at the dimmer switch on the GRAY wire.
If not, check for power at the LIGHT GREEN/YELLOW striped wire in the same connector. If you do not have power there, you have a broken wire between fuse #8 in the dash fuse box and the switch.
If you have power on the green/yellow striped wire but not on the gray wire, your dimmer switch has failed. If you have power on the gray wire out of the switch, then you have a broken wire between the switch and the dome lights. Below is the wiring diagram for your dome lights. Ford calls them the courtesy lights.