How do I turn off immobilizer, or anti-theft device?

What is your question?

Won't start, when turning key just hear a single click. All lights work, and radio, seen theft light flashing, Not sure what exactly is wrong, replaced starter, and new battery. Where should I check next, was running fine, then my interior lights started flickering while I was driving, switch for them was off. Next day van would not start.

What makes this problem better or worse?

Nothing has changed in last couple months

How long has your 2003 Ford Windstar had this problem?

About 2 months