Well, when a light stays on when the switch is off, in this case the switch is the door pin contact, it can only be one thing, a short to ground. Most people consider a short to ground to always blow a fuse, but obviously in your case it didnt. Whats happening is somewhere in the dome light circuit the wire might be frayed or cut a little and its grounding after the switch. When power grounds and by-passes the switch, it doesnt allow you to operate the circuit and the light will always stay on. Basically you have to obtain a wiring diagram and see where a possible source of the short is, and fix the wire. It might take a dealer a couple hours to find this, but in the hands of someone who knows wiring, less than one hour of labor should fix your domelight problem.
__________________ 2000 GT auto, BBK fenderwell mount intake, BBK 75mm tb, Trick Flow upper plenum, Ford Racing shorty headers, Open exhaust no mufflers, Underdrive pulleys, Intrax 2'' lowering springs, 275/40/17 out back and 225/45/17 in front. | 
Dome light stays on 
