Help --Sport Tac Fog lamps come on then the sun goes down!!! Even when vehicle off

I have a 2003 Sport Trac XLT with Auto-ON head lamps. The on-off for the fog lights is a separate switch on the dash near the radio. Normally, the fog lamps will only come on with the low beams. They are supposed to turn on automatically if they are switched on and the main headlamp switch is in the auto setting and the low beams are on. They will also come on manually if the main headlamp switch is turned "on"  and the fog lamp switch is "on" and the low beams are on. Normally the little light on the fog lamp switch glows only when the fog lamps are on.
  Here's the problem: when the sun goes down the fog lamps turn on---and kills the battery. The headlights do not come on with the fog lamps. This happens when the vehicle is turned off, the main headlamp switch can be on auto or manual, it doesn't seem to matter. Interestingly, the fog lamp switch glows "on" all the time. Turning the fog lamps on or off doesn't matter, the on/off indicator of the fog lamp switch stays glowing.
  Are there any Ford mechanics, engineers or Sport Trac experts that can shed some light on this problem?