My sisters 99 Windstar when parked and engine is turned off, after about 10 seconds taillights will come on (not brake lights) and stay untill battery is dead. No other lights come on, only rear taillights. thanks for any info.

Check all the lightbulbs in the car anytime you have strange electrical problems. I have found numerous times that a multi-filament bulb can blow a filament and the broken filament will bend and short against the other filamanet and either short or backfeed electricity. It can cause the strangest behavior.

My sisters 99 Windstar when parked and engine is turned off, after about 10 seconds taillights will come on (not brake lights) and stay untill battery is dead. No other lights come on, only rear taillights. thanks for any info.

Positive current is always present at both light on 99 and up*models. Negative is coming from the REM ( rear electronic module) for all rear lights.
When you turn on the light switch, the REM send negative to the rear parking light. So it could be a problem with the rear module or a short (if I can say) between the REM and the light at the back. This can also be caused by a trailer hitch electrical system if you have one.
Another thing to add is that your model has a system to shut down the lights after 15 or 20 minutes if you forget to shutdown them, this is to save the battery. So I would really look at the wiring harness between the REM and the rear lights.