1990 mercury, no headlights, no power anywhere, 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

Normally I'd be looking at the headlight switch as they are somewhat suspect on Fords of that vintage but you have swapped it already. You really need to get a TVMM for the car so you can see the point at which the wiring meets for the affected systems. Also check all the wiring in and around the headlight /dimmer switches also as part of the problem Fords had was overheated wiring that can cause intermittent problems.
Sorry I don't have a smoking gun soultion for you but usually these problems require a lot of digging and schematics are virtually a must. Stu

I found the problem, The wires that come off the alt. go about 18 inches then join into three more wires, I was tracing with my light probe and as soon as I picked up this jumble of wires they caught on fire. So I stripped them and soldered them together and everything was working fine. They were oily and it must have just broken down at the connection. Thanks for the input capt. I do appriciate the effprt and the responce....Bob

Glad you got it sorted out. Stu