1990 mercury, no headlights, no electrical power at all, sometimes

My ex-wifes car of course. 1990 Mercury, 5.0 a/t new battery. it would start but no headlights or tailights unless she pulled back and held onto the dimmer lever. wipers work, turn signals work, brake lights work, but no headlights or dash stuff at all. She would pull back on the lever and hold it and she had headlights. NOW, nothing, it will turn over great crossing the solinoid. but not on it's own. So I unpluged then repluged the box under the dash that resembles the computer and it fires right up, but still no headlights. So I replace the combination headlight switch for a used one from my other ford and still no headlights. after a while it wont turn over again but the battery is FULLY charged. I then replace the combination dimmer switch and turnsignal on the steering colum with a brand new motorcraft switch. Starts up and still no headlights untill I pull back on the dimmer lever, headlights work untill i shut it off again, then nothing again untill it sits for a while, then it starts but no headlights again. WTF. please help

Sounds like you have a bad conection at the starter relay or a burned fuse link. All of your power wires should start at the positive cable on the starter relay. Use a test light and make sure they all have power past the fuse links. Some times the connector at the head light dimmer switch will deform from heat and when you plug a new switch in it will push a terminal out the back side, but that doesn't explain the no start.