2003 Ford Windstar Electrical problems

It started with the interior lights coming on while I was driving. One day sitting in the driveway, the headlights started flashing by themselves and the horn started blowing. After that, while driving, the gauges went haywire, the tach never started working again, the others re intermittent. Now, the van will start but nothing inside will work, no heat/AC, no radio, no turn signals, no headlights, no hazards, no power locks/windows, no gauges. My local mechanic has had the van for a week and he has no idea.